Remove more NaN in categories checking
These can probably all be removed / simplified now that we don't allow NaN in the categories.


- https://github.com/pandas-dev/pandas/blob/38afa9310040f1bd4fb122008e96fe6d719b12a2/pandas/core/arrays/categorical.py#L1353
- https://github.com/pandas-dev/pandas/blob/38afa9310040f1bd4fb122008e96fe6d719b12a2/pandas/core/arrays/categorical.py#L1326
- https://github.com/pandas-dev/pandas/blob/38afa9310040f1bd4fb122008e96fe6d719b12a2/pandas/core/arrays/categorical.py#L1279
- https://github.com/pandas-dev/pandas/blob/38afa9310040f1bd4fb122008e96fe6d719b12a2/pandas/core/arrays/categorical.py#L1892-L1898 (maybe?)

Maybe some others if people want to look.

xref https://github.com/pandas-dev/pandas/issues/19787