bugfix: unneeded render on map request

Long description:

manage_window() will call tree_render() when appropriate, so pushing
changes to X here is not needed.

To apply this patch, use:
curl http://cr.i3wm.org/patch/582/raw.patch | git am

@@ -282,7 +282,6 @@ static void handle_map_request(xcb_map_request_event_t *event) {

     add_ignore_event(event->sequence, -1);

     manage_window(event->window, cookie, false);

-    x_push_changes(croot);

     return;

 }