output_plugin: don't pass audio_output object to method init()

audio_output_get_name() has been removed, which was the only function
left in output_api.h.  The output plugin doesn't need the audio_output
object at all, remove the parameter from the init() method.

src/output/jack_plugin.c

@@ -157,8 +157,7 @@ mpd_jack_info(const char *msg)
 #endif
 
 static void *
-mpd_jack_init(G_GNUC_UNUSED struct audio_output *ao,
-	      G_GNUC_UNUSED const struct audio_format *audio_format,
+mpd_jack_init(G_GNUC_UNUSED const struct audio_format *audio_format,
 	      const struct config_param *param)
 {
 	struct jack_data *jd;