REF: use BlockManager.apply for Rolling.count
- [ ] closes #xxxx
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Thanks @jbrockmendel. Couldn't find a straightforward simplification immediately, so this is a nice cleanup.  