Remove "DifferentialConflictsField" custom field

Summary: Ref T11114. This is a pure paring field and now entirely handled by DifferentialConflictsCommitMessageField.

Test Plan: Grepped for removed class name.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17073