Remove Differential "Title" custom field

Summary: Ref T11114. Obsoleted by Modular Transactions + EditEngine + CommitMessageField + we just "hard code" the title of revisions into the page because we're craaazy.

Test Plan:

• Made an edit on stable.
• Viewed the edit on this change, it still had the proper UI strings.
• Edited/created/updated revisions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17083