audio: Removed declarations of functions that don't exist anymore.

src/audio/SDL_sysaudio.h

@@ -331,7 +331,4 @@ extern AudioBootStrap N3DSAUDIO_bootstrap;
 extern AudioBootStrap EMSCRIPTENAUDIO_bootstrap;
 extern AudioBootStrap QSAAUDIO_bootstrap;
 
-extern SDL_AudioDevice *get_audio_dev(SDL_AudioDeviceID id);
-extern int get_max_num_audio_dev(void);
-
 #endif // SDL_sysaudio_h_