Removed swizzling of UIViewController's present(Modal)ViewController

… because it breaks UIViewController's presentingViewController property and thereby modal unwind segues.

iOS 5+ uses UIViewController's definesPresentationContext property to determine which controller is parent for a modal view controller. That system should handle presentation just fine even without swizzling. But this also requires that all parentViewControllers are set correctly in the way stated by Apple's documentation (see pull request #443).