Cleanup literal comparison ops (#863)

[ghost]

Remove unnecessary handling of py3 literal comparison ops since Literal provides total ordering.

fixes #863

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.

[coveralls]

coverage: 90.963% (+0.004%) from 90.959%
when pulling 5c9d8f9 on wallberg-umd:issue-863-cleanup-literal-comparison
into 9834665 on RDFLib:main.

[nicholascar]

Thanks very much: cleanups in code are always very welcome!