Do not make extra API calls in mailchimp_interest_groups_form_elements()

In mailchimp_interest_groups_form_elements(), $mc_lists->getInterests() is called to load up the interests via the MailChimp API, not hitting any of the usual caches. For us, this helps lead to API use overages.

These are actually already loaded up and cached and available in the existing list data, the $group variable in this function.