DEPR: remove use of `nan_as_null` from callers of `__dataframe__`
No one was using it yet and it seemed easier to clean it up from libraries than to ask everyone to add support for it - see https://github.com/data-apis/dataframe-api/issues/125.
pre-commit.ci autofix @phofl gentle ping (your review is blocking the pr)

let's get the text in, and if/when we start a formal deprecation cycle we can add the `.. deprecated::` directive and issue a warning Fine by me if we deprecated properly in the future (I don't care when we start the process) Thanks - let's get this in then

if/when we start the deprecation process we can add `.. deprecated::` and raise a warning, but for now noting that it's deprecated in the spec and has no effect seems like a net positive

adding either now might result in something accidentally enforcing the deprecation in 3.0