CLN: remove unused min_count argument in libgroupby.group_nth
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Does this have any impact to an end user? > Does this have any impact to an end user?

Good point. I *think* it allows the user to pass `min_count` to `group_nth`. Will see if I can come up with a test thanks @arw2019 