xdg-shell: remove redundant NULL buffer check

This is already checked in role precommit handler, and if the buffer is
NULL due to failed upload, that means the surface was already unmapped.

types/xdg_shell/wlr_xdg_surface.c

@@ -336,10 +336,6 @@ void handle_xdg_surface_commit(struct wlr_surface *wlr_surface) {
 		surface->mapped = true;
 		wlr_signal_emit_safe(&surface->events.map, surface);
 	}
-	if (surface->configured && !wlr_surface_has_buffer(surface->surface) &&
-			surface->mapped) {
-		unmap_xdg_surface(surface);
-	}
 }
 
 void handle_xdg_surface_precommit(struct wlr_surface *wlr_surface) {