Fix error when deleting revision

Summary: DifferentialAffectedPath has no id or phid key so delete() won't work and we have to do things this other way.

Test Plan: deleted a few diffs on my test reproduction. aside from warnings about missing keys (epriestley is on it as I write this) no errors found and diff observed as deleted

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1846

Differential Revision: https://secure.phabricator.com/D3610