Remove arcanist projects from working copy code

Summary: Ref T7604. Remove "arcanist projects" from ArcanistWorkingCopy and a few other callsites. Depends on D12999.

Test Plan: Can't really think of how to test this.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12945