Revert r316064 "Fix the incorrect detection of ICONV_LIBRARY_PATH"

This broke the use of libxml2 on machines where iconv() is provided by libc.
I'll follow up on the mailing list to discuss how to fix this properly.

> This is introduced in rL308711.
> Check for c library is incorrect here just because libc will be found always
> and it does not mean that iconv is presented.
>
> Thank to Andrew Krasny for narrowing down the root cause.
>
> Reviewers: ecbeckmann
> Reviewed By: ecbeckmann
> Subscribers: mgorny, llvm-commits
> Differential Revision: https://reviews.llvm.org/D38875

llvm-svn: 317517