Remove unused cimports
This helps a little bit with the complexity of the libs interdependencies.

Make properties.pyx its own module, since we will end up needing it to be separate when `tseries.offsets` gets moved into cython.

Remove the long-defunct _libs/src/offsets.pyx.

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