Delete binaries for other operating systems at install time

when mip is installed it comes with binaries for windows, mac, and linux. These are very large - in my experience making mip larger than even numpy or pandas!

It would be good to delete the binaries for the other operating systems during installation of the package - perhaps something inside setup.py.

This is highly relevant to serverless computing environments which often have severely limited disk space, and it's a hassle to do this manually.

Any thoughts on whether this change would be problematic? I should be able to chuck through a PR for it some time.