Remove arcanist projects from Herald
Summary: Ref T7604. Remove arcanist projects from Herald. Depends on D12894 and D12957.
Test Plan: See D12957.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: johnny-bit, Korvin, epriestley
Maniphest Tasks: T7604
Differential Revision: https://secure.phabricator.com/D12896