window: Do not handle ungrabbed events when unmanaging

Once we are no longer managing a window, we have no business in
dealing with it anymore, and operations like focusing, raising or
pinging the window aren't expected to work, and can go horribly
wrong if we try.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2467

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1676>
(cherry picked from commit e7b58c23)