libobs: Remove 'get_transition_volume' callback

This callback will no longer be used, instead transitions will modify
the audio data directly.

libobs/obs-source.h

@@ -374,15 +374,6 @@ struct obs_source_info {
 	void (*key_click)(void *data, const struct obs_key_event *event,
 			bool key_up);
 
-	/**
-	 * Called to transition sources get the volume of a transitioning
-	 * sub-source.
-	 *
-	 * @param data         Source data
-	 * @param source       Transitioning sub-source to get the volume of
-	 */
-	float (*get_transition_volume)(void *data, obs_source_t *source);
-
 	/**
 	 * Called when the filter is removed from a source
 	 *