message: [TF:TRT] Minor code clean up.
  Remove an unnecessary field in AsyncHelper.
diff: @@ -72,15 +72,11 @@ class AsyncHelper : public core::RefCounted {
  ~AsyncHelper() override { this->operator()(); }
  void operator()() {
  - if (!called_) {
  done_();
  - called_ = true;
  - }
  }
  private:
  AsyncOpKernel::DoneCallback done_;
  - bool called_ = false; // Has `done_` been called?
  };
  // This OP can construct TRTEngine on the fly and if construction of engine
  