Remove "arcanist projects" from arcanist

Summary: Ref T7604. Remove "arcanist projects". Depends on D12894.

Test Plan: This diff (so meta).

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12901