Delete EmptyObserver

It creates no value and confuses with Observers.empty() which throws an OnErrorNotImplementedException if onError is invoked.
Fixes #1741

Author: benjchristensen

src/main/java/rx/internal/operators/BufferUntilSubscriber.java

@@ -21,8 +21,7 @@
 import rx.Observer;
 import rx.Subscriber;
 import rx.functions.Action0;
-import rx.observers.EmptyObserver;
-import rx.observers.Subscribers;
+import rx.observers.Observers;
 import rx.subjects.Subject;
 import rx.subscriptions.Subscriptions;
 
@@ -51,9 +50,6 @@
  */
 public class BufferUntilSubscriber<T> extends Subject<T, T> {
 
-    @SuppressWarnings("rawtypes")
-    private final static Observer EMPTY_OBSERVER = new EmptyObserver();
-
     /**
      * @warn create() undescribed
      * @return
@@ -96,7 +92,7 @@ public void call(final Subscriber<? super T> s) {
                 s.add(Subscriptions.create(new Action0() {
                     @Override
                     public void call() {
-                        state.observerRef = EMPTY_OBSERVER;
+                        state.observerRef = Observers.empty();
                     }
                 }));
                 boolean win = false;