Remove dead-code from setup.py

[pelson]

Summary of changes

Removes the dead code in setup.py:

• the definition of package data has no effect (since in pyproject.toml we declare include-package-data=true (from https://github.com/pypa/setuptools/pull/4479)
• the lingering pypi_link function has no effect

I hesitated to also remove the os.chdir functionality, but considered that maybe this is important for something else (although unusual).

Pull Request Checklist

(not applicable)

• [x] Changes have tests
• [x] News fragment added in [newsfragments/]. (See [documentation][PR docs] for details)