Don't call beginItemDrag() for XDND

The dragged actors are not overview items.

https://bugzilla.gnome.org/show_bug.cgi?id=686984

js/ui/panel.js

@@ -739,7 +739,6 @@ const ActivitiesButton = new Lang.Class({
         if (pickedActor == this.actor) {
             if (!Main.overview.visible && !Main.overview.animationInProgress) {
                 Main.overview.showTemporarily();
-                Main.overview.beginItemDrag(actor);
             }
         }