Deprecate ordered=None for CategoricalDtype
xref https://github.com/pandas-dev/pandas/pull/26327#discussion_r282733852.

We currently allow CategoricalDtype.ordered to be None. It should just be a bool (default False).

```
In [8]: CategoricalDtype(categories=None, ordered=None).ordered
# None
```

categories can be None, which means "infer later on". But (IIRC) we don't do that for ordered.