R1:1db6b9bafdef

Remove unused PhabricatorObjectHandleConstants class

Summary: This class is no longer used.

Test Plan: `grep`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13117