Clean up some PonderAnswer / PonderAnswerQuery cruft
Summary: Ref T3578. Ref T2715. Clean up some cruft so we can use Application PHIDs for "ANSW" and eventually unbeta Ponder.
Test Plan: Grep, voted on an answer.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T2715, T3578
Differential Revision: https://secure.phabricator.com/D6580