Note that, since poll answers belong to a user and not to a voter, we
aren't doing anything regarding poll answers. This is a separate topic
that might be dealt with in a separate pull request.
Also note that, since there are no records belonging to poll voters, and
poll voters don't use `acts_as_paranoia` and don't have any callbacks on
destroy, it doesn't really matter whether we call `destroy!` or
`delete`. We're using `delete` so there are no unintended side-effects
that might affect voters with the same `user_id` and `poll_id` on
Consul Democracy installations customizing this behavior.
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