Make "global" global.

Rather than doing "let global = Shell.Global.get()" everywhere we
need it, just create a global variable called "global".

http://bugzilla.gnome.org/show_bug.cgi?id=594546

js/ui/panel.js

@@ -168,8 +168,6 @@ function Panel() {
 
 Panel.prototype = {
     _init : function() {
-        let global = Shell.Global.get();
-
 
         this.actor = new Big.Box({ orientation: Big.BoxOrientation.HORIZONTAL
                                  });