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