CLN: remove unused import causing flake8 error on master
Cleanup in #23971 removed the last use of the `copy` module, so `flake8` is now failing master due to an unused import. This fixes master by removing the import.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Hello @qwhelan! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/groupby/ops.py`](https://github.com/qwhelan/pandas/blob/403e1cd1a2ed223ebfa26aa50654265d439d08ab/pandas/core/groupby/ops.py) !

 thanks