Remove most of the legacy hunk code

Summary: Ref T8475. This gets rid of most of the old "legacy hunk" code. I'll nuke the rest (and drop the old table) once we're more sure that we're in the clear.

Test Plan: Browsed Differential.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T8475

Differential Revision: https://secure.phabricator.com/D17040