Remove leftover code for "postponed" lint and unit status

Summary: Ref T9134. It looks like this functionality was removed in D13848. Depends on D13869.

Test Plan: Ran arc diff, arc lint and arc unit.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Maniphest Tasks: T9134

Differential Revision: https://secure.phabricator.com/D13868