Differential - fix bug destroying diffs

Summary: I think this was a "hacked" sub thing that never got updated when we switched to a real editor? I am not 100% sure how these methods are used, so please let me know if I should expand my test plan. Fixes T6659.

Test Plan: made a diff from the web ui, looked up the phid from mysql, ran bin/remove destroy <phid>, and it worked!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6659

Differential Revision: https://secure.phabricator.com/D10911