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/appDisplay.js

@@ -1089,8 +1089,6 @@ AppWell.prototype = {
 
     // Draggable target interface
     acceptDrop : function(source, actor, x, y, time) {
-        let global = Shell.Global.get();
-
         let appSystem = Shell.AppSystem.get_default();
 
         let app = null;