In Swift 4.2
self.navigationController?.navigationBar.setBackgroundImage(UIImage(), for: .default)
self.navigationController?.navigationBar.shadowImage = UIImage()
self.navigationController?.navigationBar.isTranslucent = true
(в viewWillAppear), а затем в viewWillDisappear, чтобы отменить его, поставьте
self.navigationController?.navigationBar.shadowImage = nil
self.navigationController?.navigationBar.isTranslucent = false