Remove deprecated files and functionalities

[pietermarsman]

Pull request

Closes #973

Closes #736

Remove depracted files and functionalities.

How Has This Been Tested?

Running nox.

Checklist

• [x] I have read CONTRIBUTING.md.
• [x] I have added a concise human-readable description of the change to CHANGELOG.md.
• [x] I have tested that this fix is effective or that this feature works.
• [x] I have added docstrings to newly created methods and classes.
• [x] I have updated the README.md and the readthedocs documentation. Or verified that this is not necessary.