dnd: Don't queue an idle handler if we already have one

Removing an existing source before scheduling a new one is not wrong,
but slightly less effective than doing nothing and relying on the
previously created source to do the job.

https://bugzilla.gnome.org/show_bug.cgi?id=711555

js/ui/dnd.js

@@ -391,7 +391,7 @@ const _Draggable = new Lang.Class({
 
     _queueUpdateDragHover: function() {
         if (this._updateHoverId)
-            GLib.source_remove(this._updateHoverId);
+            return;
 
         this._updateHoverId = GLib.idle_add(GLib.PRIORITY_DEFAULT,
                                             Lang.bind(this, this._updateDragHover));