PERF: avoid object array cast in pd.isna
- [x] closes #18856
- [ ] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#18856 is a fairly old issue about avoiding an object cast in the `_isna` method in `core/dtypes/missing.py`. AFAICT we never hit that clause in the method in our current testing suite. Modulo CI turning something up we should just delete it

Update: this *does* get hit. Looking into the ideas in #18856