Remove weird integration between Legalpad and the ExternalAccount table

Summary:
Depends on D20107. Ref T6703. Legalpad currently inserts "email" records into the external account table, but they're never used for anything and nothing else references them.

They also aren't necessary for anything important to work, and the only effect they have is making the UI say "External Account" instead of "None" under the "Account" column. In particular, the signatures still record the actual email address.

Stop doing this, remove all the references, and destroy all the rows.

(Long ago, Maniphest may also have done this, but no longer does. Nuance/Gatekeeper use a more modern and more suitable "ExternalObject" thing that I initially started adapting here before realizing that Legalpad doesn't actually care about this data.)

Test Plan: Signed documents with an email address, saw signature reflected properly in UI. Grepped for other callsites.

Reviewers: amckinley

Reviewed By: amckinley

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T6703

Differential Revision: https://secure.phabricator.com/D20108