lib: remove unused msg parameter in debug_agent

Removed the msg parameter in the Client function
of _debug_agent.js, because it is unused.

PR-URL: https://github.com/nodejs/node/pull/11833


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Jackson Tian <shyvo1987@gmail.com>