gh-36893: get rid of one ParentWithGens
mirco-ticket to get rid of one `ParentWithGens`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/36893
Reported by: Frédéric Chapoton
Reviewer(s): Travis Scrimshaw
