Remove "DifferentialParentRevisionsField" custom field

Summary: Ref T11114. This was obsoleted by UI changes and hacked around for performance in T11404. It no longer does anything.

Test Plan: Grepped for removed class name.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17077