Solution checked and work in Swift 5
Below I put few solutions for different cases:
1. Remove text from back button
The best solution to remove text from back button is to add in viewDidLoad():
navigationItem.backBarButtonItem = UIBarButtonItem()
2. Set own text on back button
In case you want to set your own title, do it by setting title of backButton:
let backButton = UIBarButtonItem()
backButton.title = "My Title"
navigationItem.backBarButtonItem = backItem
3. Empty back button on all VC
If you want to create common style in entire app - to have just arrow back without text, create base VC for all your View Controllers:
class BaseViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        navigationItem.backBarButtonItem = UIBarButtonItem()
    }
}
Solution presented above let you customize back button in the future if you want to make some exception later, by adding additional variable and overriding it in specific ViewController, f.ex:
class BaseViewController: UIViewController {
    var customBackButtonTitle: String?
    override func viewDidLoad() {
        super.viewDidLoad()
        var backButton = UIBarButtonItem()
        if let text = customBackButtonTitle {
            backButton.title = text
        }
        navigationItem.backBarButtonItem = backButton
    }
}