Completely remove "differential.find" Conduit API method

Summary:
Ref T2543. I believe there have been no upstream callsites of this method since D1646, in February 2012.

The method works, and we can revert this if needbe, but this seems like a good time to remove support.

Test Plan: Grepped for differential.find.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18397