MetaMTA - make sure mail garbage collection also cleans up recipient edges

Summary: Ref T5791. This edge table grows 2+X faster than the corresponding mail table depending on usage. Ergo, lets make sure to clean that up too in the delete code.

Test Plan: careful thought

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5791

Differential Revision: https://secure.phabricator.com/D13408