Remove Proof from HybridNew, as it was removed from RooStats

[gpetruc]

Removing support for Proof in HybridNew calculator, as it was removed from RooFit and RooStats in root-project/root#14993 , so that the code still compiles on the ROOT master branch (tested on the dev3 LCG nightly build)
I don't think anyone ever used in the past ten years (maybe not even before), and from the ROOT PR my understanding is that it was anyway not functional since several root releases.

@guitargeek

[guitargeek]

Thanks for making these adjustments! Proof needs so serialize the likelihood with ROOT IO to send it to the different workers, and I disabled ROOT IO for the likelihood classes already in ROOT 6.28, accidentally breaking PROOF support in RooFit:

• [RF] Make old test statistic classes transient with ClassDef(0) root-project/root#11909

As nobody has complained about that since then, I think we can safely conclude that it was unused.

If we need parallelization to speed up something in RooFit (like the generation of toys), it should be re-implemented with modern C++ approaches like std::thread or what RDataFrame does (building on top of TBB).

So: LGTM for this PR from my side.