coreaudio: we don't need to track number of allocated audio buffers anymore.

CoreAudio takes care of iterating through the buffers and freeing them now,
so we don't have to manage this ourselves.

src/audio/coreaudio/SDL_coreaudio.h

@@ -47,7 +47,6 @@ struct SDL_PrivateAudioData
 {
     SDL_Thread *thread;
     AudioQueueRef audioQueue;
-    int numAudioBuffers;
     AudioQueueBufferRef *audioBuffer;
     void *buffer;
     UInt32 bufferOffset;