src: remove unused fields isolate_

Currently the following compiler warnings are generated:

In file included from ../src/node_platform.cc:1:
../src/node_platform.h:83:16:
warning: private field 'isolate_' is not used [-Wunused-private-field]
  v8::Isolate* isolate_;
               ^
1 warning generated.

This commit removes these unused private member.

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>