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