Revert "[cmake] Enable zlib support on windows"

This reverts commit r319533 as it broke llvm-config --system-libs output
and everything that depends on it (which is mostly out of tree or
downstream folks, but includes a couple of llvm buildbots as well).

I think I have a fix for this in D40779, but I want someone to look
review it first. In the mean time, I am reverting this change, as it
seems to break a lot of people.

llvm-svn: 319663