Remove "async lint" from arc lint

Summary: Ref T12996. This was a very old FB-specific feature that caused some kind of server-side lint thing to run. You can/should do this now with Harbormaster instead. Nothing else ever used it and it has no role in the upstream.

Test Plan: Grepped for asynclint, created this revision.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T12996

Differential Revision: https://secure.phabricator.com/D18641