Policy - remove loadRepository() method from ArcanistProjects

Summary: Ref T7094. This loadRepository() method bypassed policy unnecessarily. kill it.

Test Plan: basically un-tested since arcanist projects are deprecated and the main callsites were in releeph. conduit end point still works though!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11586