remove cython extension.

Now in cython package itself, as stable.
remove Cython Built-in extension notebook
remove more ref to cythonmagic
use Cython.load_ipython_ext to simplify
add whatsnew and uppercase C in load_ext

Use uppercase as this is what is in cython docs.
Fix version string comparison, capitalisation and typos.
Merge pull request #6510 from Carreau/removecython

Remove cython extension and examples.