Port GDM and Caribou to GDBus

During the mass port to GDBus, this classes were left out (probably
because they didn't exist at the time). Now it's time to update
them.

https://bugzilla.gnome.org/show_bug.cgi?id=664436

js/ui/status/accessibility.js

@@ -1,6 +1,5 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
-const DBus = imports.dbus;
 const GDesktopEnums = imports.gi.GDesktopEnums;
 const Gio = imports.gi.Gio;
 const Gtk = imports.gi.Gtk;