Remove atime from fields tracked

Summary: We don't get notified on access (too expensive), so remove atime from the list of fields tracked. It's misleading at best, dangerous at worst.

Test Plan: `arc unit --everything`

Reviewers: wez

Reviewed By: wez

Differential Revision: https://phabricator.fb.com/D873778

--HG--
extra : amend_source : a730972b29b6553fa2ab2481498431130d7f3383