Remove arcanist project datasource

Summary: Ref T7604. This class is no longer used.

Test Plan: grep

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T7604

Differential Revision: https://secure.phabricator.com/D13004