Don't explicitly name abstract base classes
Summary: Ref T5655. It is superfluous to include "base" in the name of an abstract base class. Furthermore, it is not done consistently within the code base.
Test Plan: Ran arc unit.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: epriestley, Korvin
Maniphest Tasks: T5655
Differential Revision: https://secure.phabricator.com/D9989