remove unused block attribute
grepping across the code, `_downcast_dtype` doesn't show up anywhere else.

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

ping on green. After 11 hours Travis hasn't started.  I think they've had some recent infrastructure problems; may need to be cancelled and re-started. thanks.

yeah travis was really borked yesterday / today.