output_thread: fix double lock

During the whole output thread, the audio_output object is locked, and
it is only unlocked while waiting for the GCond and while running a
plugin method.  The error handler in ao_play_chunk() attempted to lock
the object again, which was code from MPD 0.15.x which should have
been removed a long time ago.

src/output_thread.c

@@ -463,12 +463,9 @@ ao_play_chunk(struct audio_output *ao, const struct music_chunk *chunk)
 
 			/* don't automatically reopen this device for
 			   10 seconds */
-			g_mutex_lock(ao->mutex);
-
 			assert(ao->fail_timer == NULL);
 			ao->fail_timer = g_timer_new();
 
-			g_mutex_unlock(ao->mutex);
 			return false;
 		}