debugger: remove obsolete _debug_agent.js

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

PR-URL: #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>

Author: Trott

lib/_debug_agent.js

@@ -22,7 +22,6 @@
     'node_core_target_name%': 'node',
     'library_files': [
       'lib/internal/bootstrap_node.js',
-      'lib/_debug_agent.js',
       'lib/assert.js',
       'lib/buffer.js',
       'lib/child_process.js',