Remove duplication from Whitehall.searchable_classes

Both SupportingPage and DocumentCollection are now edition classes,
so do not need to be present in additional_classes
Time to get rid of the old supporting_pages table

SupportingPage was converted to an Edition sub-class last year. Now
that the migration of old supporting pages to the editions table is
complete, we can drop this legacy table.
Merge pull request #1300 from alphagov/cleanup-after-editionification

Cleanup after editionification