Remove deprecated "base" classes

Summary: Ref T5655. After D9983, ArcanistBaseWorkflow and ArcanistBaseUnitTestEngine are deprecated.

Test Plan: Wait a sufficient amount of time before landing this.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D10004