Remove remaining arcanist project code

Summary: Fixes T7604. This is the big scary change which drops the "arcanist project" fields from the database permanently.

Test Plan:
grepped for the following to ensure that I had found all remaining references:

• /arcanistProject/i
• /arcanist_project/i
• /projectName/i
• /project_name/i
• /project_id/i

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D12899