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