Changes slug setter to put slugs on a solr document field named 'slug', ...

...instead of 'id', and leaves the 'id' field alone.

Removes the addition of a 'slug' datastream on Fedora objects. It's not being used.
As I had originally written it, this could apply a slug for the first time, or reslug an asset that already had a slug: in the latter case, `old_slug` would be given, and it wouldn't be a problem if we found it... in fact we'd expect to find it.

If we still want to be able to do that, then we probably still want the `and !old_slug`
... or, if not, we should get rid of the `old_slug` variable?

I could be confused.