Revert r351324 "Build LLVM-C.dll by default on windows and enable in release package"

This broke the build, ending up with too long command-lines when invoking gen-mscv-exports.py.

> As it says in the subject, should have gone long enough now that this
> should be safe. This will greatly simplify dealing with LLVM for people
> that just want to use the C API on windows. This is a follow up from
> D35077.
>
> Patch by Jakob Bornecrantz!
>
> Differential revision: https://reviews.llvm.org/D56774

llvm-svn: 351329