Revert "src: fix -Wunused-variable compiler warning"

This reverts commit f938ef7.

Refs: nodejs#6382
Refs: nodejs#6204
Refs: nodejs#5834

Author: Fishrock123

src/handle_wrap.cc

@@ -38,6 +38,7 @@ void HandleWrap::Unref(const FunctionCallbackInfo<Value>& args) {
 
 
 void HandleWrap::IsRefed(const FunctionCallbackInfo<Value>& args) {
+  Environment* env = Environment::GetCurrent(args);
   HandleWrap* wrap = Unwrap<HandleWrap>(args.Holder());
 
   bool refed = IsAlive(wrap) && (wrap->flags_ & kUnref) == 0;