PyUtils: ROOT6Setup cleanup and add batch mode

Several changes to the PyUtils.Helpers.ROOT6Setup:

• Remove no longer needed Python2 backwards compatibilty code
• Revert XML import workarounds (!31320 (merged),!31424 (merged)) for ATEAM-597 as those are no longer needed in LCG_98.
• With the new PyROOT the command line argument -b is no longer parsed to set the batch mode. Add an option to ROOT6Setup to set batch mode whenever the first ROOT import is done. This can then be used by athena.py or athenaHLT.py (ATR-21890) to set the batch mode but without having to actually import ROOT early in the boostrap.

cc @mnowak @wlampl @ssnyder