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: #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>

src/node_platform.cc

@@ -65,7 +65,7 @@ size_t BackgroundTaskRunner::NumberOfAvailableBackgroundThreads() const {
 
 PerIsolatePlatformData::PerIsolatePlatformData(
     v8::Isolate* isolate, uv_loop_t* loop)
-  : isolate_(isolate), loop_(loop) {
+  : loop_(loop) {
   flush_tasks_ = new uv_async_t();
   CHECK_EQ(0, uv_async_init(loop, flush_tasks_, FlushTasks));
   flush_tasks_->data = static_cast<void*>(this);

src/node_platform.h

@@ -80,7 +80,6 @@ class PerIsolatePlatformData :
   static void RunForegroundTask(uv_timer_t* timer);
 
   int ref_count_ = 1;
-  v8::Isolate* isolate_;
   uv_loop_t* const loop_;
   uv_async_t* flush_tasks_ = nullptr;
   TaskQueue<v8::Task> foreground_tasks_;