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