js: Drop unused Gdk/Gtk imports

There's nothing from those modules used in those JS files.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/317

js/ui/main.js

@@ -1,7 +1,6 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
 const Clutter = imports.gi.Clutter;
-const Gdk = imports.gi.Gdk;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Gtk = imports.gi.Gtk;