R1:93c0ffd02c82

Remove "Child Revisions" custom field in Differential

Summary: Ref T11114. This was obsoleted by the "Stack" graph and does nothing.

Test Plan: Viewed revisions, still saw dependency graphs.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11114

Differential Revision: https://secure.phabricator.com/D17072