removed purge-questionnaire task

_only merge if you **don't** want the purge functionality_

There is a task to clear out the old unclaimed questionnaires (where someone has clicked 'create questionnaire', but hasn't signed up to claim it within the last 12 hours).

This task hasn't ever run on production,  though will do when the rake deploy tasks are fixed in #639.

This will change the counts on the [/status](https://certificates.theodi.org/status) page.  Currently they include certificates that haven't been claimed by a user.

If this is an important thing to keep track of - then *merge* this PR to remove the purge task.  This needs to be done before #639 is resolved (cc/ @pezholio & @pikesley)