shell: Drop XDND initialization code

Let mutter take over this, and drop this piece of backend-specific
code.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/611

js/ui/xdndHandler.js

@@ -17,9 +17,6 @@ var XdndHandler = class {
         Main.uiGroup.add_actor(this._dummy);
         this._dummy.hide();
 
-        if (!Meta.is_wayland_compositor())
-            global.init_xdnd();
-
         var dnd = Meta.get_backend().get_dnd();
         dnd.connect('dnd-enter', this._onEnter.bind(this));
         dnd.connect('dnd-position-change', this._onPositionChanged.bind(this));