Revert "Add fallback for registry entries without slugs."

Since the slugs are now migrated, we don't need this fallback any more.

This reverts commit ca08b45af9bfeac7e89d13e14b70c056a109afd3.
Merge pull request #149 from alphagov/remove-slug-fallback

Revert "Add fallback for registry entries without slugs."