Remove unused internal Observer types (#582)

Rx.NET/Source/src/System.Reactive/Internal/Observers.cs

@@ -14,91 +14,4 @@ namespace System.Reactive
 
         public void OnNext(T value) { }
     }
-
-    internal sealed class DoneObserver<T> : IObserver<T>
-    {
-        public static readonly IObserver<T> Completed = new DoneObserver<T>();
-
-        public Exception Exception { get; set; }
-
-        public void OnCompleted() { }
-
-        public void OnError(Exception error) { }
-
-        public void OnNext(T value) { }
-    }
-
-    internal sealed class DisposedObserver<T> : IObserver<T>
-    {
-        public static readonly IObserver<T> Instance = new DisposedObserver<T>();
-
-        public void OnCompleted()
-        {
-            throw new ObjectDisposedException("");
-        }
-
-        public void OnError(Exception error)
-        {
-            throw new ObjectDisposedException("");
-        }
-
-        public void OnNext(T value)
-        {
-            throw new ObjectDisposedException("");
-        }
-    }
-
-    internal sealed class Observer<T> : IObserver<T>
-    {
-        private readonly ImmutableList<IObserver<T>> _observers;
-
-        public Observer(ImmutableList<IObserver<T>> observers)
-        {
-            _observers = observers;
-        }
-
-        public void OnCompleted()
-        {
-            foreach (var observer in _observers.Data)
-            {
-                observer.OnCompleted();
-            }
-        }
-
-        public void OnError(Exception error)
-        {
-            foreach (var observer in _observers.Data)
-            {
-                observer.OnError(error);
-            }
-        }
-
-        public void OnNext(T value)
-        {
-            foreach (var observer in _observers.Data)
-            {
-                observer.OnNext(value);
-            }
-        }
-
-        internal IObserver<T> Add(IObserver<T> observer) => new Observer<T>(_observers.Add(observer));
-
-        internal IObserver<T> Remove(IObserver<T> observer)
-        {
-            var i = Array.IndexOf(_observers.Data, observer);
-            if (i < 0)
-            {
-                return this;
-            }
-
-            if (_observers.Data.Length == 2)
-            {
-                return _observers.Data[1 - i];
-            }
-            else
-            {
-                return new Observer<T>(_observers.Remove(observer));
-            }
-        }
-    }
 }