Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Setuptools was updated and our hacky namespace-package-chickens came
back to roost. This removes the unsupported namespace package hacks.

Since we no longer reference `run_codegen.py` in the Quick Start for
Python, remove it from the repository to avoid confusion.

This should have been included in 46585e23.

Fix #5619

Fixes #4122