Remove "arc lint --only-changed"

Summary:
Ref T12996. See also T9749. This option is the opposite of --lintall, and means "don't show warnings on changed lines".

However, it's confusing (and I think no one uses it), and we can reasonably infer what you mean if you give us --rev. Simplify this logic a bit and choose the "all / only changed" behavior based on the only sensible thing we can reasonably do given the flags.

As with all the other cruft here, I'm pretty sure this came from FB.

Test Plan: Grepped for only-changed, lintAll, created this revision.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12996

Differential Revision: https://secure.phabricator.com/D18644