gh-36709: get rid of coerce_try
remove the deprecated `_coerce_try` method in `parent_old`
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
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URL: https://github.com/sagemath/sage/pull/36709
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
