setup.sh shouldn't set PYTHONPATH

[kjmeagher]

In older version of cvmfs it was important to set PYTHONPATH, but in newer versions the search path is compiled into the python binary and now PYTHONPATH is not only useless but in some cases hurtful. see https://icecube-spno.slack.com/archives/C02KQL9KN/p1716826647734099

[dsschult]

While I agree, I messed with this for over a week in py3-v4.3.0 and couldn't get it to work reliably (outside venvs) without setting it. It's something to look into for the next version.