src: remove unused using decls

PR-URL: https://github.com/nodejs/node/pull/7990


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yorkie Liu <yorkiefixer@gmail.com>