xdg-shell: Remove the serial from popup_done

It doesn't serve any purpose, as it's a serial that the client gave to
the server when starting the popup, which the client already has.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

clients/window.c

@@ -4727,7 +4727,7 @@ menu_redraw_handler(struct widget *widget, void *data)
 }
 
 static void
-handle_popup_popup_done(void *data, struct xdg_popup *xdg_popup, uint32_t serial)
+handle_popup_popup_done(void *data, struct xdg_popup *xdg_popup)
 {
 	struct window *window = data;
 	struct menu *menu = window->main_surface->widget->user_data;