Allow removal of deleted announcements from index

Destroying statistics announcements raises an exception with real rummager because it cannot generate the link to remove it from the index. This is because the :statistics_announcement_dates association is now a dependent and gets destroyed when the announcement is destroyed, but the current announcement date is delegated to when generating the searchable content to get the link.

This is a follow up to https://github.com/alphagov/whitehall/pull/1678