Delete unused code in DelaySubscription.swift

The parent didn't used in DelaySubscriptionSink

RxSwift/Observables/DelaySubscription.swift

@@ -26,14 +26,6 @@ extension ObservableType {
 final private class DelaySubscriptionSink<O: ObserverType>
  : Sink<O>, ObserverType {
  typealias E = O.E
- typealias Parent = DelaySubscription<E>
-
- private let _parent: Parent
-
- init(parent: Parent, observer: O, cancel: Cancelable) {
- self._parent = parent
- super.init(observer: observer, cancel: cancel)
- }
 
  func on(_ event: Event<E>) {
  self.forwardOn(event)
@@ -56,7 +48,7 @@ final private class DelaySubscription<Element>: Producer<Element> {
  }
 
  override func run<O : ObserverType>(_ observer: O, cancel: Cancelable) -> (sink: Disposable, subscription: Disposable) where O.E == Element {
- let sink = DelaySubscriptionSink(parent: self, observer: observer, cancel: cancel)
+ let sink = DelaySubscriptionSink(observer: observer, cancel: cancel)
  let subscription = self._scheduler.scheduleRelative((), dueTime: self._dueTime) { _ in
  return self._source.subscribe(sink)
  }