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