Remove the direct dependency of MVAUtils to TMVA

In principle the BDT representation used by MVAutils does no depend on TMVA.

TMVA is needed for "converting" between formats.

Move out the relevant code from the BDT class itself to a helper.

@akraszna explained to me that we use --as-needed when linking, so by not using anything that depends on TMVA on what is compiled for the library the TMVA should not be part of its dependencies.

To confirm I also checked the .so of the MVAUtils library via ldd and I do not see TMVA. I see it for the executable under utils (for which the TMVA/XML/MathCore are needed).

Also removed some of the ROOT dependencies that seemed unneeded.