Swap duplicate close status on Ponder for invalid

Summary: Until we have a proper close as duplicate workflow for Ponder, remove the option with something more sensible.

Test Plan: Closed a question as invalid, saw it closed and in feed.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14007