REGR: fix DataFrame sum and prod with min_count and numeric_only
- [ ] closes #41074
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

it appears that the clean-up in #35899 to simplify _reduce may have been a bit premature, have reinstated a few lines of the removed code for backport purposes.

The longer term fix would probably be to support min_count in _mgr.reduce, https://github.com/pandas-dev/pandas/pull/40143#issuecomment-787945291