Remove arcanist projects from differential

Summary: Ref T7604. Remove arcanist projects from differential. Depends on D12687 and D12893.

Test Plan: Submitted a diff. Patched the diff with arc patch.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12894