Remove arcanist projects from DiffusionRequest

Summary: Ref T7604. This data is no longer used. Depends on D12894.

Test Plan: grep

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12895