Allow file comments to be edited

Summary:
Fixes T10750. Files have some outdated cache/key code which prevents recording an edit history on file comments.

Remove this ancient cruft.

(Users must bin/storage adjust after upgrading to this patch to reap the benefits.)

Test Plan:

• Ran bin/storage adjust.
• Edited a comment in Files.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10750

Differential Revision: https://secure.phabricator.com/D16312