Remove unsupported system_packages from RTD config

[atugushev]

• https://blog.readthedocs.com/drop-support-system-packages/
• Deprecation: remove "use system packages" (python.system_packages config key and UI checkbox) readthedocs/readthedocs.org#10562

Contributor checklist

• Included tests for the changes.
• PR title is short, clear, and ready to be included in the user-facing changelog.

Maintainer checklist

• Verified one of these labels is present: backwards incompatible, feature, enhancement, deprecation, bug, dependency, docs or skip-changelog as they determine changelog listing.
• Assign the PR to an existing or new milestone for the target version (following Semantic Versioning).

[atugushev]

Thanks @theryanwalker!