Revert "place child clients above fullscreen clients"

This does not work as intended. Lets revert it temporarily and add it back
after the release.

This reverts commit 298949bbc4.

client.h

@@ -183,18 +183,6 @@ client_get_parent(Client *c)
 	return p;
 }
 
-static inline int
-client_has_children(Client *c)
-{
-#ifdef XWAYLAND
-	if (client_is_x11(c))
-		return !wl_list_empty(&c->surface.xwayland->children);
-#endif
-	/* surface.xdg->link is never empty because it always contains at least the
-	 * surface itself. */
-	return wl_list_length(&c->surface.xdg->link) > 1;
-}
-
 static inline const char *
 client_get_title(Client *c)
 {