Revert "Add window title to app_ids for different"

This reverts commit 39eab4bf44472790dddf02490ac260f810c30c9a.

From @Kontrabant:
Making windows use the window title as the app ID by default doesn't seem like the right direction. The app ID is used by window managers to group windows from the same application together for switching between with alt-tilde and such functionality, and giving each window it's own app ID would break this.

src/video/wayland/SDL_waylandwindow.c

@@ -1270,11 +1270,7 @@ void Wayland_ShowWindow(_THIS, SDL_Window *window)
             }
         } else {
             data->shell_surface.xdg.roleobj.toplevel = xdg_surface_get_toplevel(data->shell_surface.xdg.surface);
-            if (c->classname != NULL) {
-                xdg_toplevel_set_app_id(data->shell_surface.xdg.roleobj.toplevel, c->classname);
-            } else {
-                xdg_toplevel_set_app_id(data->shell_surface.xdg.roleobj.toplevel, window->title);
-            }
+            xdg_toplevel_set_app_id(data->shell_surface.xdg.roleobj.toplevel, c->classname);
             xdg_toplevel_add_listener(data->shell_surface.xdg.roleobj.toplevel, &toplevel_listener_xdg, data);
         }
     }