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