self.navigationBar.setBackgroundImage(UIImage(), forBarMetrics: UIBarMetrics.Default)
self.navigationBar.shadowImage = UIImage()
self.navigationBar.translucent = true
swift版:
[self.navigationBar setBackgroundImage:[UIImage new]
forBarMetrics:UIBarMetricsDefault];
self.navigationBar.shadowImage = [UIImage new];
self.navigationBar.translucent = YES;