message: chore(subscriber): remove _parentSubsciption
diff: @@ -47,8 +47,6 @@ export class Subscriber<T> extends Subscription implements Observer<T> {
  protected isStopped: boolean = false;
  protected destination: PartialObserver<any> | Subscriber<any>; // this `any` is the escape hatch to erase extra type param (e.g. R)
  - private _parentSubscription: Subscription | null = null;
  -
  /**
  * @param {Observer|function(value: T): void} [destinationOrNext] A partially
  * defined Observer or a `next` callback function.
  @@ -161,7 +159,6 @@ export class Subscriber<T> extends Subscription implements Observer<T> {
  this.isStopped = false;
  this._parent = _parent;
  this._parents = _parents;
  - this._parentSubscription = null;
  return this;
  }
  }
  