ddof parameter on pl.corr is nonsensical

[orlp]

I only realized this after implementing it recently, but the ddof parameter is not used for pl.corr at all. It completely cancels out of the formula. To confirm this in np.corrcoef you'll see:

ddof: _NoValue, optional
    Has no effect, do not use.

We should just remove the parameter in the Rust code and on the Python side deprecate the parameter, removing it in 2.0.

[orlp]

The same holds for pl.rolling_corr.

[zachlefevre]

I am interested in doing this work as my first contribution to Polars.