director: Avoid "ring sync timeout" errors when all backends are down

doveadm commands were failing with it. Also pending request failures were
logged as failing due to ring sync timeout, instead of because no hosts.

src/director/main.c

@@ -226,7 +226,7 @@ static void director_state_changed(struct director *dir)
 	ARRAY(struct director_request *) new_requests;
 	bool ret;
 
-	if (!dir->ring_synced || !mail_hosts_have_usable(dir->mail_hosts))
+	if (!dir->ring_synced)
 		return;
 
 	/* if there are any pending client requests, finish them now */