src: fix -Wunused-variable compiler warning

Introduced in commit 7d8882b, "handle_wrap: expose an `isRefed()` check
to JS".

PR-URL: https://github.com/nodejs/node/pull/6129
Reviewed-By: Evan Lucas <evanlucas@me.com>

src/handle_wrap.cc

@@ -39,7 +39,6 @@ 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;