Remove unsupported `system_packages` from RTD config

[atugushev]

• https://blog.readthedocs.com/drop-support-system-packages/
• https://github.com/readthedocs/readthedocs.org/pull/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

• [x] 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!