Remove compatibility with 3.1

Created by: Zaharid

As per https://docs.nnpdf.science/releases.html#id4 having published the 4.0 paper we no longer need to support 3.1-era runcards and fits. To some approximation that means rm -rf nnpdfcpp but I think also things like the complicated backwards compatibility support for experiments (which has been very useful) would no longer be needed, leading to some opportunities for simplification.

It would be probably good to tag the code with the last version supporting 3.1 compatibility.