Remove the concept of a secondary search index.

This was a temporary hack to mix together mainstream results and
detailed guidance: since these are now presented as separate
result streams (and if we do join them back together it'll be in
a very different way), we can remove secondary search.

The main search endpoint now just becomes a special case of the
`/backend_name/search` endpoint, where the backend name is blank.
We also don't need explicit references to the primary search in the
controller code any more.
Mark the NullBackend class for removal.

Once we've cleared all references to it out of the deployment code, we
can delete the class.
Remove the remnants of auto-complete.

Since the only implementation of auto-complete was in the Solr code
that we removed, there's no point in keeping this code around.
Be explicit about only supporting JSON.
Merge pull request #59 from alphagov/remove-secondary-search

Remove secondary search (and autocomplete)