src: remove unused isolate variable

Currently the following compiler warning is generated:
../src/node_task_queue.cc:93:12: warning:
unused variable 'isolate' [-Wunused-variable]
  Isolate* isolate = env->isolate();
           ^
1 warning generated.

This commit removes the unused variable.

PR-URL: nodejs#25368
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: danbev

src/node_task_queue.cc

@@ -90,7 +90,6 @@ void PromiseRejectCallback(PromiseRejectMessage message) {
 static void SetPromiseRejectCallback(
     const FunctionCallbackInfo<Value>& args) {
   Environment* env = Environment::GetCurrent(args);
-  Isolate* isolate = env->isolate();
 
   CHECK(args[0]->IsFunction());
   env->set_promise_reject_callback(args[0].As<Function>());