gh-36689: less use of _an_element_impl
replace some methods `_an_element_impl` by `_an_element_`
one little step on the long way towards getting rid of `parent_old`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/36689
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
