Remove the hack to check UNAME_RELEASE when identifying the Darwin version.

This was only needed to locate llvm-gcc's installation directory when clang
falls back to run llvm-gcc for i386 kexts.  As of clang svn r140187, we're
now just searching paths with several different Darwin versions on either
side of the current version, so this is no longer needed.

llvm-svn: 140188