src: remove unused fields msg_ and env_

Currently the following compiler warnings are generated:

../src/node_messaging.cc:74:16:
warning: private field 'env_' is not used [-Wunused-private-field]
  Environment* env_;
               ^
../src/node_messaging.cc:75:12:
warning: private field 'msg_' is not used [-Wunused-private-field]
  Message* msg_;
           ^
2 warnings generated.

This commit removes these unused private members.

PR-URL: https://github.com/nodejs/node/pull/20876


Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Shingo Inoue <leko.noor@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>
Reviewed-By: John-David Dalton <john.david.dalton@gmail.com>
Reviewed-By: Gus Caplan <me@gus.host>