refactor grouper __iter__
- [x] closes #52468 
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Removed the `__iter__` overload that was introduced in #51414. The logic accessing the `grouping_vector` for was introduced #53088
Thanks @taytzehao!