boxpointer: Don't use boxpointer actor, as it's now an actor itself

Remove this.actor = actor, since the class is now an actor itself.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/487

js/misc/util.js

@@ -366,7 +366,7 @@ class CloseButton extends St.Button {
     }
 
     _computeBoxPointerOffset() {
-        if (!this._boxPointer || !this._boxPointer.actor.get_stage())
+        if (!this._boxPointer || !this._boxPointer.get_stage())
             return 0;
 
         let side = this._boxPointer.arrowSide;