Remove "Revision ID" custom field

Summary: Ref T11114. Obsoleted by DifferentialRevisionIDCommitMessageField.

Test Plan:

• Grepped for removed class.
• Created a new revision, verified that the amended message included a proper revision ID.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17080