The swift counterpart for your objective-c code is this
let appDomain = NSBundle.mainBundle().bundleIdentifier!
NSUserDefaults.standardUserDefaults().removePersistentDomainForName(appDomain)
Swift 3.0 and higher
let appDomain = Bundle.main.bundleIdentifier!
UserDefaults.standard.removePersistentDomain(forName: appDomain)
