[TMVA] Annotate variable with [[maybe_unused]]

[dpiparo]

to avoid warnings about unused variable. This is possible because in master c++17 is enforced.

[phsft-bot]

Starting build on ROOT-performance-centos8-multicore/soversion, ROOT-ubuntu2204/nortcxxmod, ROOT-ubuntu2004/python3, mac11/noimt, mac12arm/cxx20, windows10/default
How to customize builds

[phsft-bot]

Build failed on windows10/default.
Running on null:C:\build\workspace\root-pullrequests-build
See console output.

Failing tests:

• projectroot.bindings.pyroot.pythonizations.test.pyunittests_pyroot_pyz_rtensor
• projectroot.roottest.root.meta.roottest_root_meta_loadernotapp_build
• projectroot.roottest.root.meta.roottest_root_meta_loadernotapp
• projectroot.roottest.root.meta.genreflex.ROOT-5881.roottest_root_meta_genreflex_ROOT_5881_writeTreeBinary_build
• projectroot.roottest.root.meta.genreflex.ROOT-5881.roottest_root_meta_genreflex_ROOT_5881_execStringInputFiles
• projectroot.roottest.root.meta.tclass.issue-6840.roottest_root_meta_tclass_issue_6840_emulatePairs_build

[phsft-bot]

Build failed on ROOT-ubuntu2004/python3.
Running on root-ubuntu-2004-1.cern.ch:/home/sftnight/build/workspace/root-pullrequests-build
See console output.

Failing tests:

• projectroot.tmva.pymva.test.PyMVA_Keras_Regression