Allow herald rules to be deleted with `./bin/remove destroy`.

Summary: Fixes T5286. Allow herald rules to be deleted using the `./bin/remove destroy` workflow.

Test Plan: Created a herald rule. Deleted it with `./bin/remove destroy`.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T5286

Differential Revision: https://secure.phabricator.com/D9416