core: drop pointless assert()

We checked this literally two lines earlier, no point in checking so
quickly again.

src/core/manager.c

@@ -1281,7 +1281,6 @@ static unsigned manager_dispatch_stop_when_unneeded_queue(Manager *m) {
 
         while ((u = m->stop_when_unneeded_queue)) {
                 _cleanup_(sd_bus_error_free) sd_bus_error error = SD_BUS_ERROR_NULL;
-                assert(m->stop_when_unneeded_queue);
 
                 assert(u->in_stop_when_unneeded_queue);
                 LIST_REMOVE(stop_when_unneeded_queue, m->stop_when_unneeded_queue, u);