Remove vestigal AuxiliaryField code from Maniphest

Summary: Gets rid of as much of this as possible. We'll batch handles and remarkup again some day, but after ApplicationTransactions.

Test Plan: Edited, viewed, and checked email for custom field edits.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7037