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