Make AuthProvider, ExternalAccount, and ExternalAccountIdentifier all Destructible

Summary: Depends on D21014. Ref T13493. Make these objects all use destructible interfaces and destroy sub-objects appropriately.

Test Plan:

• Used bin/remove destroy --trace ... to destroy a provider, a user, and an external account.
• Observed destruction of sub-objects, including external account identifiers.

Maniphest Tasks: T13493

Differential Revision: https://secure.phabricator.com/D21015