layout: Remove an unused variable

The tray barrier was removed when the hot corner to activate the message
tray was removed.

https://bugzilla.gnome.org/show_bug.cgi?id=677215

js/ui/layout.js

@@ -114,7 +114,6 @@ const LayoutManager = new Lang.Class({
         this._background = null;
         this._leftPanelBarrier = 0;
         this._rightPanelBarrier = 0;
-        this._trayBarrier = 0;
 
         this._inOverview = false;
         this._updateRegionIdle = 0;