removing vim-ipython, since it has it's own repo

Previously, vim-ipython was being update "by hand" just before a release. I'm
not sure it makes sense to ship vim-ipython with IPython proper anymore (since
it currently includes a bunch of code for workaround of the drift in API and
messaging spec)

If folks feel differently, I can make an alternative PR where the examples/vim
directory is a submodule pointing to vim-ipython