util: Remove unused imports

https://bugzilla.gnome.org/show_bug.cgi?id=679944

js/misc/util.js

@@ -1,9 +1,6 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
-const Gdk = imports.gi.Gdk;
-const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
-const Shell = imports.gi.Shell;
 
 const Main = imports.ui.main;