Revert "Add an invalid #define for SDL_ThreadID() to SDL_oldnames.h (#15801)"

This reverts commit b04d026458cd762db3499bec54348e51acd76508.

This caused problems building sdl2-compat:
https://github.com/libsdl-org/sdl2-compat/actions/runs/27378527337/job/81120423041

docs/README-migration.md

@@ -41,8 +41,6 @@ Many functions and symbols have been renamed. We have provided a handy Python sc
 rename_symbols.py --all-symbols source_code_path
 ```
 
-**Note** that `rename_symbols.py` can't completely handle `SDL_ThreadID`, see the [SDL_Thread.h section](#sdl_threadh) below.
-
 It's also possible to apply a semantic patch to migrate more easily to SDL3: [SDL_migration.cocci](https://github.com/libsdl-org/SDL/blob/main/build-scripts/SDL_migration.cocci)
 
 SDL headers should now be included as `#include <SDL3/SDL.h>`. Typically that's the only SDL header you'll need in your application unless you are using OpenGL or Vulkan functionality. SDL_image, SDL_mixer, SDL_net, SDL_ttf and SDL_rtf have also their preferred include path changed: for SDL_image, it becomes `#include <SDL3_image/SDL_image.h>`. We have provided a handy Python script [rename_headers.py](https://github.com/libsdl-org/SDL/blob/main/build-scripts/rename_headers.py) to rename SDL2 headers to their SDL3 counterparts:
@@ -2048,15 +2046,6 @@ The following functions have been removed:
 The following symbols have been renamed:
 * SDL_threadID => SDL_ThreadID
 
-### Attention: Potential problems with `SDL_ThreadID`
-
-**_`SDL_ThreadID`_** is a case that needs **special attention**, because if you forget to rename a call to `SDL_ThreadID()` to `SDL_GetCurrentThreadID()` in **C++** code, it will *still compile fine*, calling the default constructor of the `SDL_ThreadID` type, initializing it to `0` - but if `SDL_GetCurrentThreadID()` was intended, that of course leads to **wrong behavior** that can be hard to catch.
-
-Furthermore **`rename_symbols.py` does not handle it** (because it would be hard to do in a way that doesn't break anything when running that script twice), which makes it even more error-prone.
-
-SDL_oldnames.h now has a `#define` that should catch these cases, but it's not available in SDL 3.4.10 and older. So if you're using 3.4.10 or older, check your code carefully for occurences of `SDL_ThreadID()` and make sure it wasn't supposed to be `SDL_GetCurrentThreadID()`.
-In some edge cases this `#define` can cause false positives (compile errors even though `SDL_ThreadID` was used correctly); right above that definition in the header is a long comment explaining possible problems and solutions.
-
 ## SDL_timer.h
 
 SDL_GetTicks() now returns a 64-bit value. Instead of using the SDL_TICKS_PASSED macro, you can directly compare tick values, e.g.

include/SDL3/SDL_oldnames.h

@@ -1336,58 +1336,4 @@
 
 #endif /* SDL_ENABLE_OLD_NAMES */
 
-/* In SDL2, `SDL_ThreadID()` was a function that is now called `SDL_GetCurrentThreadID()`.
- * In SDL3, the thread ID *type* is called `SDL_ThreadID` (in SDL2 it was
- * `SDL_threadID` with lower 't').
- *
- * Unfortunately, at least in C++ writing SDL_ThreadID() compiles fine even for
- * the type, it's the default constructor of the type, so e.g.
- * `myID = SDL_ThreadID();` is equivalent to `myID = 0;`
- *
- * Of course if you've been porting SDL2 code and have missed renaming this case
- * of `SDL_ThreadID()` to `SDL_GetCurrentThreadID()` this is quite a pitfall:
- * The code compiles fine, but behaves wrong: it will set `myID` to `0` instead
- * of the current thread's ID.
- *
- * This makes it impossible to provide a "proper" `SDL_ENABLE_OLD_NAMES` treatment
- * of SDL_ThreadID() in this header, so the following `#define` sets `SDL_ThreadID()`
- * to an invalid value, forcing you to consciously adjust it to your needs, i.e.
- * replacing it with `SDL_GetCurrentThreadID()` or `0` depending on your intentions.
- *
- * Note that using `SDL_ThreadID x = ...;` is unaffected, only "SDL_ThreadID" followed
- * by "(" will lead to a compile error.
- *
- * Unfortunately, there are some cases where this breaks legit SDL3 code that
- * actually means to use `SDL_ThreadID(...`, like:
- * - Definitions of function pointers with SDL_ThreadID as return type, like
- *   `typedef SDL_ThreadID (SDLCALL *SDL_DYNAPIFN_SDL_GetCurrentThreadID) (void);`
- * - C++ casts like `SDL_ThreadID tid = SDL_ThreadID(id);`
- *
- * For C++ casts, just replace `SDL_ThreadID(id)` with either a C-style cast
- * ( `(SDL_ThreadID)id` ) or one of the other C++ cast types like
- * `static_cast<SDL_ThreadID>(id)`.
- *
- * For the (hopefully rare) case of function pointers returning SDL_ThreadID,
- * or any other legit case where `SDL_ThreadID` is followed by `(` that can't be
- * easily avoided by using slightly different syntax, you can `#undef SDL_ThreadID`
- * before your code using it to get rid of this define causing the compiler error.
- *
- * If you're using `SDL_ENABLE_OLD_NAMES` to support both SDL2 and SDL3 with
- * the same code, consider adding:
- *
- * ```c
- * #if SDL_MAJOR_VERSION == 2
- *   #define SDL_GetCurrentThreadID() SDL_ThreadID()
- * #endif
- * ```
- *
- * and using `SDL_GetCurrentThreadID()` in your code, even if you otherwise use
- * the SDL2 names.
- *
- * The gain of catching the bugs caused by accidentally using `SDL_ThreadID()`
- * when `SDL_GetCurrentThreadID()` was intended hopefully outweight the annoyances
- * caused by this in some rare cases.
- */
-#define SDL_ThreadID()  SDL_ThreadID_renamed_SDL_GetCurrentThreadID
-
 #endif /* SDL_oldnames_h_ */

src/dynapi/SDL_dynapi.c

@@ -55,12 +55,6 @@
 #include <windows.h>
 #endif
 
-#ifdef SDL_ThreadID
-/* prevent the SDL_ThreadID() define from SDL_oldnames.h from breaking
-   function pointer definitions */
-#undef SDL_ThreadID
-#endif
-
 /* This is the version of the dynamic API. This doesn't match the SDL version
    and should not change until there's been a major revamp in API/ABI.
    So 2.0.5 adds functions over 2.0.4? This number doesn't change;