src: remove unused variable

PR-URL: #18385
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>

src/node_perf.cc

@@ -7,7 +7,6 @@ namespace node {
 namespace performance {
 
 using v8::Array;
-using v8::ArrayBuffer;
 using v8::Context;
 using v8::Function;
 using v8::FunctionCallbackInfo;
@@ -324,7 +323,6 @@ void Init(Local<Object> target,
   Environment* env = Environment::GetCurrent(context);
   Isolate* isolate = env->isolate();
   performance_state* state = env->performance_state();
-  auto state_ab = ArrayBuffer::New(isolate, state, sizeof(*state));
 
   target->Set(context,
               FIXED_ONE_BYTE_STRING(isolate, "observerCounts"),