R1:44b17d0a9ace

Remove unused ManiphestTaskOwner class

Summary: This class is unused after D12526.

Test Plan: `grep`

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12562