CLN: remove unused categories/ordered handling in astype
This removes some unused code in the internals `astype`. There is a small change in behaviour though, when passing the categories/ordered keyword you now get TypeError instead of ValueError. But since that is Python's default behaviour, I would say this is rather a good fix.
No opinion on the TypeError vs ValueError.  Looks like a nice cleanup. Shouldn't this get an entry in the whatsnew ("Removal of prior version deprecations/changes"). else no comment. > Shouldn't this get an entry in the whatsnew ("Removal of prior version deprecations/changes").

It was already removed before. It was only not "fully" removed, and instead of just removing kwargs, we explicitly raised an error if categories or ordered was passed as a kwarg (instead of relying on python to raise that error).
 lgtm. thanks @jorisvandenbossche 