Remove inline declarations in pxd files
At the moment (I think since .26, not sure) they don't do anything but cause lots of warnings during cythonizing.  Eventually they will raise errors.

https://github.com/cython/cython/issues/1706#issuecomment-302347613

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