client: run client_manager_expire() in an idle event

With the GLib main loop, the client manager can install its own event
in case a client is expired.  No need for main.c to call
client_manager_expire() manually.

src/main.c

@@ -196,7 +196,6 @@ main_notify_triggered(void)
 	unsigned flags;
 
 	syncPlayerAndPlaylist();
-	client_manager_expire();
 	reap_update_task();
 
 	/* send "idle" notificaions to all subscribed