gnome-shell-plugin: Remove old code to detect crossing events

Ever since we stopped reparenting status icon windows to the stage, tray icons
haven't been inferiors and thus we don't have to filter out enter/leave events
for them, as we never show them outside of a modal.

https://bugzilla.gnome.org/show_bug.cgi?id=706536
shell-gtk-embed: Make the MetaWindowActor for a tray icon unreactive

The MetaWindowActor isn't painted, and we empty its input shape in
the X scene graph, but Clutter still picked it. Set it as unreactive
so that it can't be picked.

https://bugzilla.gnome.org/show_bug.cgi?id=706536