R1:1df84168efd9

Remove DifferentialRevisionEditor

Summary: Ref T2222. This has no callsites and no functionality not present in the TransactionEditor.

Test Plan: awwyiss

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8456