Remove some remnants of the old ways commit mesage fields worked from Differential

Summary:
Ref T11114. Ref T12085. I missed a few pieces of cleanup when moving all this stuff over.

In particular, load all fields which use Custom Field storage before doing commit-message-related stuff, instead of just the ones that claim they appear on commit messages.

Test Plan: Edited revisions and made API calls without apparent issues. See followup on T12085, shortly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12085, T11114

Differential Revision: https://secure.phabricator.com/D17207