CLN: Remove special handling of nans in the float64-case of isin
It is no longer needed because the hash-table handles the nans correctly out of the box (see #21866)

Not having to scan the values via isna(...).any()  will improve the performance.


- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

There are no changes visible for the user (others than slightly better performance).
