backgroundMenu: Don't add an animation to the background menu

We don't do this for shell entries nor for the message tray
context menu, so it doesn't make sense to do it here, either.

js/ui/backgroundMenu.js

@@ -38,7 +38,7 @@ function addBackgroundMenu(actor) {
     function openMenu() {
         let [x, y] = global.get_pointer();
         cursor.set_position(x, y);
-        actor._backgroundMenu.open(BoxPointer.PopupAnimation.FULL);
+        actor._backgroundMenu.open(BoxPointer.PopupAnimation.NONE);
     }
 
     let clickAction = new Clutter.ClickAction();