bugfix: unneeded render on unmap

Long description:

tree_render() will call x_push_changes(), so calling it afterward is not
needed.

To apply this patch, use:
curl http://cr.i3wm.org/patch/581/raw.patch | git am

@@ -487,7 +487,6 @@ static void handle_unmap_notify_event(xcb_unmap_notify_event_t *event) {

     tree_close(con, DONT_KILL_WINDOW, false, false);

     tree_render();

-    x_push_changes(croot);

 ignore_end:

     /* If the client (as opposed to i3) destroyed or unmapped a window, an