Eliminate PYTHONPATH and python38 references in openEO helm chart docs

[soxofaan]

Stumbled on this one from EOEPCA/deployment-guide#8:

The openEO webapp driver Sparkpplication helm chart usage docs show setting PYTHONPATH, containing static python38 references, e.g.

openeo-geotrellis-kubernetes/kubernetes/charts/sparkapplication/README.md

Line 41 in d67111d

PYTHONPATH: $PYTHONPATH:/opt/tensorflow/python38/2.3.0/:/opt/openeo/lib/python3.8/site-packages/

I think these things should be baked in internally into the image, and the user of the image should not be bothered with that.

Moreover, we want to upgrade from python 3.8 and these of hard-coded references make that quite hard and annoying to roll out

[soxofaan]

this should also be addressed in https://github.com/EOEPCA/deployment-guide/blob/main/docs/eoepca/openeo-geotrellis.md

[jdries]

Note that I may have solved this already when working on the images, but the yaml file is indeed still the old approach.