media-session: don't emit metadata remove in shutdown

When in shutdown we destroy all objects but don't remove all
the metadata associated with them, like route settings.

src/examples/media-session/media-session.h

@@ -228,6 +228,7 @@ struct sm_media_session_events {
 	void (*remove) (void *data, struct sm_object *object);
 
 	void (*rescan) (void *data, int seq);
+	void (*shutdown) (void *data);
 	void (*destroy) (void *data);
 };