Clean: Categorical.fillna NaN in categories checking
We don't allow NaN in the categories anymore, so this block should be unreachable.

https://github.com/pandas-dev/pandas/blob/8bfcddc7728deaf8e840416d83c8feda86630d27/pandas/core/arrays/categorical.py#L1622-L1628

If anyone wants to remove it and test things out.