strip: remove experimental.prunestrip option marmoute decided to drop it because it's "neither documented nor a good idea". This extensions was assemble to deal with Facebook willingness to check `hg strip` semantic for its user. Changing `hg strip` semantic is overall a pretty bad idea, because `stripping` is a specific operation, that is problematic in distributed version control but have a narrow set of valid usecase (eg: maintenance). Diverting user away from `prune` in favor of a modified `strip` is a bad idea, because it mask the legitimate `strip` usage and introduce confusion in user heads, The verb is not appropriate and the behavior of `prunestrip` does not match the one of vanillia `strip`. Facebook is no longer using this, and the lack of documentation is a good hint that nobody else is. So lets drop it.