BUG: as_index=False can return a MultiIndex in groupby.apply
- [x] closes #58291
- [x] [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.

Removed the addition of multi index, instead of using `reset_index()`, not sure if there is a use case that would require using `reset_index()` instead.  Thank you for the review, I have updated the pr according to the comments
 Thanks @undermyumbrella1!