Fix Apple performance frequency to match nanosecond counter

The counter returns ns via clock_gettime_nsec_np, so the frequency must be SDL_NS_PER_SECOND.

src/timer/unix/SDL_systimer.c

@@ -128,10 +128,7 @@ Uint64 SDL_GetPerformanceFrequency(void)
 #ifdef HAVE_CLOCK_GETTIME
         return SDL_NS_PER_SECOND;
 #elif defined(SDL_PLATFORM_APPLE)
-        Uint64 freq = mach_base_info.denom;
-        freq *= SDL_NS_PER_SECOND;
-        freq /= mach_base_info.numer;
-        return freq;
+        return SDL_NS_PER_SECOND;
 #endif
     }