Remove test_groupby_agg_extension (marked xfail)
Remove special `test_groupby_agg_extension` in `test_json.py` as we can now pass the test generally with these fixes to `GroupBy.first` and `GroupBy.last`.

- [x] closes #39098
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

In the course of winnowing away unnecessary changes in a PR related to extension arrays, I found that this was one useful result of that work.  I'm re-submitting as a fresh PR that specifically fixes a single reported bug, following guidance in that bug report, and what I've learned along the way. Thanks @MichaelTiemannOSC 