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