Use DestructionEngine to destroy UserEmail objects

Summary: Ref T13444. Prepare to hook identity updates when user email addreses are destroyed.

Test Plan:

• Destroyed a user with bin/remove destroy ... --trace, saw email deleted.
• Destroyed an email from the web UI, saw email deleted.

Maniphest Tasks: T13444

Differential Revision: https://secure.phabricator.com/D20912