Remove call to undefined method DoorkeeperDAO::getObjectKey()

Summary: Remove call to undefined static method DoorKeeperDAO::getObjectKey() in the DoorkeeperExternalObject class. getObjectKey() does not exist (and never existed) in any of its parent classes (DoorkeeperDAO, PhabricatorLiskDAO, LiskDAO).

Test Plan: Run static code analysis. Grep the source code and check the git history of parent classes.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25734