fs: remove unused parameter for encodeRealpathResult

The third parameter `err` is not used anywhere.

PR-URL: https://github.com/nodejs/node/pull/10862


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>