Revert "fix flickering when resizing/spawning windows"

This reverts commit 017bb7d752.

Bug: https://github.com/djpohly/dwl/issues/349

client.h

@@ -240,21 +240,6 @@ client_is_mapped(Client *c)
 	return c->surface.xdg->mapped;
 }
 
-static inline int
-client_is_rendered_on_mon(Client *c, Monitor *m)
-{
-	/* This is needed for when you don't want to check formal assignment,
-	 * but rather actual displaying of the pixels.
-	 * Usually VISIBLEON suffices and is also faster. */
-	struct wlr_surface_output *s;
-	if (!c->scene->node.enabled)
-		return 0;
-	wl_list_for_each(s, &client_surface(c)->current_outputs, link)
-		if (s->output == m->wlr_output)
-			return 1;
-	return 0;
-}
-
 static inline int
 client_is_unmanaged(Client *c)
 {