Investigate no longer using PYTHONPATH

[mferrera]

A separate utility we use enables using RMS environments without patching them in with PYTHONPATH, as long as the *PATHs are constructed correct: a slightly tricky thing.

Removing the use of PYTHONPATH would make integration in various software deployments simpler as most expect for *PATH environment variables to be changed, but less commonly look at PYTHONPATH. Hence PYTHONPATH may stick around for longer than expected, causing unexpected behavior.