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/unlockDialog.js

@@ -7,7 +7,6 @@ const Gdm  = imports.gi.Gdm;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const GnomeDesktop = imports.gi.GnomeDesktop;
-const Gtk = imports.gi.Gtk;
 const Meta = imports.gi.Meta;
 const Signals = imports.signals;
 const Shell = imports.gi.Shell;