Remove "arc lint --only-new"
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.
Reviewed By: amckinley
Differential Revision: https://secure.phabricator.com/D18642