Remove more unused variables in various areas

[hahnjo]

First delete nop method TMVA::SVWorkingSet::PrintStat(), then remove many more unused variables in various areas.

Note: If you absolutely require your part of this PR be split out, we can discuss but it's going to be work that I'd like to avoid if possible.

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds

[phsft-bot]

Build failed on ROOT-performance-centos8-multicore/cxx17. Running on olbdw-01.cern.ch:/data/sftnight/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.runtutorials.tutorial_spectrum_Background_decr
• projectroot.runtutorials.tutorial_spectrum_Background_compton

[phsft-bot]

Build failed on mac11/cxx14. Running on macphsft23.dyndns.cern.ch:/Users/sftnight/build/workspace/root-pullrequests-build See console output.

Failing tests:

• projectroot.tree.dataframe.test.gtest_tree_dataframe_test_dataframe_snapshot

[hahnjo]

@lmoneta @pcanal this PR for the math/ / tmva/ parts as well as io/

[pcanal]

@hahnjo which of the variable in core/io were you concerned about?

[hahnjo]

@hahnjo which of the variable in core/io were you concerned about?

The ones you commented on, at least these instances felt odd...

[phsft-bot]

Starting build on ROOT-debian10-i386/soversion, ROOT-performance-centos8-multicore/cxx17, ROOT-ubuntu18.04/nortcxxmod, ROOT-ubuntu2004/python3, mac1015/cxx17, mac11/cxx14, windows10/cxx14 How to customize builds