Remove the actor itself from reviewer list in commandeering

Summary: When the commandeerer was a reviewer, after the commandeering, he stayed as a reviewer. He can no longer amend the diff. He has to go 'Edit Revision' to remove himself/herself. The fix is to remove it automatically.

Test Plan:
comamndeereded a revision and the behavior is correct now:

• I was removed from the revision list
• the comment transaction shows one more entry that I removed myself as a reviewer

Reviewers: epriestley, vrana

Reviewed By: vrana

CC: nh, vrana, aran, Korvin

Maniphest Tasks: T1225

Differential Revision: https://secure.phabricator.com/D2872