Allow Ponder questions and answers to be destroyed with bin/remove

Summary: Ref T5847.

Test Plan: Used bin/remove destroy to destroy a question. Saw the question and its answers get wiped out.

Reviewers: btrahan, shadowhand

Reviewed By: shadowhand

Subscribers: shadowhand, epriestley

Maniphest Tasks: T5847

Differential Revision: https://secure.phabricator.com/D10345