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