Migration to cppyy, removal in some places where it is apparently not
needed.  Some packages modified might have been broken anyhow since the
ROOT6 migration - if they happen to be still in use, they will need
extra work to fix.
This is cherry-picked d1f78e60 [formerly ed45654d] from
master to solve ATEAM-436


Former-commit-id: 3e1eb566