Remove unused and odd code.

This code was never being used and any use of it would look fairly strange.
For example, it would try to map a NativeReaderError::file_malformed to
std::errc::invalid_argument.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@210913 91177308-0d34-0410-b5e6-96231b3b80d8