xdg-shell: stop clearing wlr_xdg_surface state on unmap

The protocol doesn't say we should, so let's not.

Also it's pointless to reset scheduled_serial, since 0 is a valid
serial.

types/xdg_shell/wlr_xdg_surface.c

@@ -85,10 +85,6 @@ void unmap_xdg_surface(struct wlr_xdg_surface *surface) {
 		wl_event_source_remove(surface->configure_idle);
 		surface->configure_idle = NULL;
 	}
-	surface->scheduled_serial = 0;
-
-	memset(&surface->current, 0, sizeof(struct wlr_xdg_surface_state));
-	memset(&surface->pending, 0, sizeof(struct wlr_xdg_surface_state));
 }
 
 static void xdg_surface_handle_ack_configure(struct wl_client *client,