Fix visibility of the PhabricatorWorker::doWork() methods

Summary: Ref T6822. This method is only called from within the PhabricatorWorker::executeTask() and PhabricatorWorker::scheduleTask() methods.

Test Plan: grepped for ->doWork.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11406