Remove some dead code in PhabricatorRepositoryRefEngine
Summary: `$class` are not read anymore since the removal of the call to `PhabricatorWorker::scheduleTask()` in rP3cb543ef.
Test Plan: Read/grep code.
Reviewers: O1 Blessed Committers, mainframe98
Reviewed By: O1 Blessed Committers, mainframe98
Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno
Differential Revision: https://we.phorge.it/D26384