Use PhabricatorObjectListQuery in Herald worker

Summary: Ref T2222. Straightforward, just breaks a needless dependency.

Test Plan: Pushed and parsed a commit with "Auditors" in it.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8473