PyJobTransformsCore: delete majority of legacy transform code

The only remaining use-cases are:

• EvgenJobTransforms
• the standalone command line utilities find_data.py, find_library.py and find_python_modules.py

Once these are removed we can delete the entire package.

Doing this in main since other cleanups (e.g. AthFile) have happened in the meantime to this obsolete code that would cause conflicts.

cc @tadej @tsulaia