cleanup: Clean up unused imports

Spotted by eslint.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/399

js/ui/sessionMode.js

@@ -1,6 +1,5 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
-const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
 const Mainloop = imports.mainloop;
 const Signals = imports.signals;