shell: Don't allow maximized surfaces to be moved with touch

Moving a maximized surface with the pointer is already not possible,
so make the behavior with touch consistent.

https://bugs.freedesktop.org/show_bug.cgi?id=78208

desktop-shell/shell.c

@@ -1453,7 +1453,7 @@ surface_touch_move(struct shell_surface *shsurf, struct weston_seat *seat)
 	if (!shsurf)
 		return -1;
 
-	if (shsurf->state.fullscreen)
+	if (shsurf->state.fullscreen || shsurf->state.maximized)
 		return 0;
 
 	move = malloc(sizeof *move);