Remove nonfunctional AJAX embed behavior for Slowvote

Summary:
See https://hackerone.com/reports/434116. Slowvote has a piece of Javascript that attempts to let you vote on {V123} polls inline.

It does not work: nothing ever triggers it (nothing renders a control with a slowvote-option sigil).

At least for now, just remove it. It has a completely separate pathway in the controller and both pathways are buggy, so this makes fixing them easier.

Test Plan: Voted in plurality and approval polls via Slowvote and the embedded widget.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19773