wxMSW propagates accelerators to the top menu in wxMenu::UpdateAccel(),
but the reverse operation in wxMenu::DoRemove() didn't do it, resulting
in leaked leftover accelerator entries that could prevent the same
accelerator from working if an item using it was later added. Fix by
adding RemoveAccel() helper method that behaves analogously to
UpdateAccel().
(backport of 60542745f6 from master)