Remove obsolete (and now incorrect) fallback replacement of realpath(3)

Remove obsolete (and now incorrect) fallback replacement of realpath(3)

The fallback replacement of realpath(3) (from 8e918ccc) uses readlink(3)
under the assumption that we’re only interested about symlinks, but
that’s no longer the case: we’re using it for normalization purposes as
well. Let’s just remove it. If it turns out that there still are
non-Windows systems that don’t have realpath(3) and that we care about
we’ll figure out something else.