REF: de-duplicate skipna logic
De-duplicate logic in handling skipna
inside methods any() and all().

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

Closing as the logic was actually slightly different in the both methods.