Start removing cython extension

Cython magic is now part of cython repo itself, which as been released as 0.21, there is though no more needs to ship it with IPython.

One of the only reason to keep it would be to support people with old version of Cython, though it is still a single file, so can easily be reused.

One more question, the cython magic is under BSD as IPython is, so the move to Cython package implies thay have to carry a copy of our licence. Shoudl I ping the current 11 authors of the cython magic file to waive the need to carry IPython licence inside cython repo (which is apache licensed) ?
Rather than knowing what the Cython extension does, we should just be calling `Cython.load_ipython_extension(ip)`, so it does exactly the same as `%load_ext Cython` for as long as we leave it here.