test: remove unused disposed_ variable

Currently building test_inspector_socket.cc generates the following
warning:
../test/cctest/test_inspector_socket.cc:189:8: warning:
private field 'disposed_' is not used [-Wunused-private-field]
  bool disposed_ = false;
       ^

1 warning generated.

This commit removes this variable.

PR-URL: https://github.com/nodejs/node/pull/17628


Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>