debugger: remove obsolete _debug_agent.js

With _debugger.js removed, _debug_agent.js appears to be unnecessary.

PR-URL: https://github.com/nodejs/node/pull/12582


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>