Clean up unused read the docs configuration and dependencies.

[rtibbles]

Summary

Description of the change(s) you made

Our developer docs are in pure markdown with no build step, so we can clean up our documentation dependencies.
Cleans them up and removes the superfluous readthedocs configuration.

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Reviewer's Checklist

This section is for reviewers to fill out.

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md