Removed SDL_MAIN_AVAILABLE on platforms that don't benefit from it

We already set the main thread when creating a thread or calling SDL_Init(), and applications can call SDL_SetMainReady() themselves for special cases, so there isn't a lot of value of forcing everyone through the SDL_RunApp() flow.

Fixes https://github.com/libsdl-org/SDL/issues/14580

include/SDL3/SDL_main.h

@@ -228,15 +228,6 @@
         */
         #define SDL_MAIN_AVAILABLE
 
-    #else
-        /*
-          This platform SDL provides a main function that sets up the main
-          thread and calls your main function.
-
-          If you provide this yourself, you may define SDL_MAIN_HANDLED
-        */
-        #define SDL_MAIN_AVAILABLE
-
     #endif
 #endif /* SDL_MAIN_HANDLED */