CMake: Removed unused variable (thanks, Felix!).

"MACOSX_COREAUDIO" is actually an internal #define set up elsewhere, and
this CMake var is never exported past the CMake script anyhow.

Partially fixes Bugzilla #2807.

CMakeLists.txt

@@ -1051,7 +1051,6 @@ elseif(APPLE)
 
   if(SDL_AUDIO)
     set(SDL_AUDIO_DRIVER_COREAUDIO 1)
-    set(MACOSX_COREAUDIO 1)
     file(GLOB AUDIO_SOURCES ${SDL2_SOURCE_DIR}/src/audio/coreaudio/*.c)
     set(SOURCE_FILES ${SOURCE_FILES} ${AUDIO_SOURCES})
     set(HAVE_SDL_AUDIO TRUE)