Remove "arc lint --only-new"

Summary:
Ref T12996. Fixes T9749. This is a very old Facebook-specific thing which relies on other server-side Facebook-specific things and doesn't work properly anyway.

I don't actually remember what the use case for this flag was. It was either "the codebase has a million warnings, so showing warnings on files/lines you touched isn't good enough", or "weird warnings that raise lint on other lines", or some variation of those.

Regardless, I believe this feature benefits at most one install (Facebook circa 2011), and likely zero. It occasionally confuses normal users.

T9749 suggests a possible replacement workflow which is likely more practical, but I'd like to see a real problem description before considering this again.

Test Plan: Created this revision, grepped for only-new.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12996, T9749

Differential Revision: https://secure.phabricator.com/D18642