Fix overly defensive handling of src parameter in make_relative_symlink

This method is for internal use only. It is unsuitable for use in
formulae, which should use install_symlink to create relative symlinks.
Thus callers are required to pass a Pathname, not a string, and we can
remove this conditional.

Further, if src is not absolute, then src.relative_path_from(dirname)
will fail. All callers currently pass absolute pathnames. Therefore we
don't need to call expand_path when printing it.