Revert "audio: Added SDL_SetAudioIterationCallbacks()."

This reverts commit 608f706a95.

Didn't end up using this in SDL3_mixer, and it's a super-awkward API if we
don't need it.

I _might_ bite the bullet and let people lock a physical audio device, though,
as I could see that being useful but less awkward for the same reasons I
originally wanted it.

include/SDL3/SDL_audio.h

@@ -2035,85 +2035,6 @@ extern SDL_DECLSPEC void SDLCALL SDL_DestroyAudioStream(SDL_AudioStream *stream)
  */
 extern SDL_DECLSPEC SDL_AudioStream * SDLCALL SDL_OpenAudioDeviceStream(SDL_AudioDeviceID devid, const SDL_AudioSpec *spec, SDL_AudioStreamCallback callback, void *userdata);
 
-/**
- * A callback that fires around an audio device's processing work.
- *
- * This callback fires when a logical audio device is about to start accessing
- * its bound audio streams, and fires again when it has finished accessing
- * them. It covers the range of one "iteration" of the audio device.
- *
- * It can be useful to use this callback to update state that must apply to
- * all bound audio streams atomically: to make sure state changes don't happen
- * while half of the streams are already processed for the latest audio
- * buffer.
- *
- * This callback should run as quickly as possible and not block for any
- * significant time, as this callback delays submission of data to the audio
- * device, which can cause audio playback problems. This callback delays all
- * audio processing across a single physical audio device: all its logical
- * devices and all bound audio streams. Use it carefully.
- *
- * \param userdata a pointer provided by the app through
- *                 SDL_SetAudioPostmixCallback, for its own use.
- * \param devid the audio device this callback is running for.
- * \param start true if this is the start of the iteration, false if the end.
- *
- * \threadsafety This will run from a background thread owned by SDL. The
- *               application is responsible for locking resources the callback
- *               touches that need to be protected.
- *
- * \since This datatype is available since SDL 3.4.0.
- *
- * \sa SDL_SetAudioIterationCallbacks
- */
-typedef void (SDLCALL *SDL_AudioIterationCallback)(void *userdata, SDL_AudioDeviceID devid, bool start);
-
-/**
- * Set callbacks that fire around a new iteration of audio device processing.
- *
- * Two callbacks are provided here: one that runs when a device is about to
- * process its bound audio streams, and another that runs when the device has
- * finished processing them.
- *
- * These callbacks can run at any time, and from any thread; if you need to
- * serialize access to your app's data, you should provide and use a mutex or
- * other synchronization device.
- *
- * Generally these callbacks are used to apply state that applies to multiple
- * bound audio streams, with a guarantee that the audio device's thread isn't
- * halfway through processing them. Generally a finer-grained lock through
- * SDL_LockAudioStream() is more appropriate.
- *
- * The callbacks are extremely time-sensitive; the callback should do the
- * least amount of work possible and return as quickly as it can. The longer
- * the callback runs, the higher the risk of audio dropouts or other problems.
- *
- * This function will block until the audio device is in between iterations,
- * so any existing callback that might be running will finish before this
- * function sets the new callback and returns.
- *
- * Physical devices do not accept these callbacks, only logical devices
- * created through SDL_OpenAudioDevice() can be.
- *
- * Setting a NULL callback function disables any previously-set callback.
- * Either callback may be NULL, and the same callback is permitted to be used
- * for both.
- *
- * \param devid the ID of an opened audio device.
- * \param start a callback function to be called at the start of an iteration.
- *              Can be NULL.
- * \param end a callback function to be called at the end of an iteration. Can
- *            be NULL.
- * \param userdata app-controlled pointer passed to callback. Can be NULL.
- * \returns true on success or false on failure; call SDL_GetError() for more
- *          information.
- *
- * \threadsafety It is safe to call this function from any thread.
- *
- * \since This function is available since SDL 3.4.0.
- */
-extern SDL_DECLSPEC bool SDLCALL SDL_SetAudioIterationCallbacks(SDL_AudioDeviceID devid, SDL_AudioIterationCallback start, SDL_AudioIterationCallback end, void *userdata);
-
 /**
  * A callback that fires when data is about to be fed to an audio device.
  *