overview: Remove some unused variables

Those are left-overs from the custom swipe-scroll implementation.

https://bugzilla.gnome.org/show_bug.cgi?id=686639

js/ui/overview.js

@@ -133,9 +133,6 @@ const Overview = new Lang.Class({
         this._backgroundGroup.hide();
         this._bgManagers = [];
 
-        this._capturedEventId = 0;
-        this._buttonPressId = 0;
-
         this.visible = false;           // animating to overview, in overview, animating out
         this._shown = false;            // show() and not hide()
         this._shownTemporarily = false; // showTemporarily() and not hideTemporarily()