strip: drop -b/--backup option (BC)

This option had very limited utility and counterintuitive behavior and
collided unfortunately with the much later -B option.

Normally we would no-op such a feature so as to avoid annoying existing
scripts. However, we have to weigh that against the silent misbehavior
that results when users mistakenly intended to use -B: because -b
takes no arg, the bookmark gets interpreted as a normal revision, and
gets stripped without removing the associated bookmark, while also not
backing up the revision in question. A no-op behavior or warning would only
remove the latter half of the misadventure.

The only users I can find of this feature were using it in error and
have since stopped. The few (if any) remaining users of this feature
would be better served by --no-backup.

--HG--
extra : amend_source : 8e4cac7ec63677eee3904dfb8eb96b9b84e687d7