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