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/notificationDaemon.js

@@ -4,7 +4,6 @@ const Clutter = imports.gi.Clutter;
 const GdkPixbuf = imports.gi.GdkPixbuf;
 const Gio = imports.gi.Gio;
 const GLib = imports.gi.GLib;
-const Gtk = imports.gi.Gtk;
 const Shell = imports.gi.Shell;
 const Mainloop = imports.mainloop;
 const St = imports.gi.St;