Allow Arcanist Projects to be deleted using ./bin/remove destroy.

Summary: Self-explanatory.

Test Plan: Created (and destroyed) an arcanist project. Verified that the deletion actually happened.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T4749

Differential Revision: https://secure.phabricator.com/D9353