=str #16986 Fix memory leak in PrefixAndTail when using Sink.publisher

The problem was reproduced with the TCK PrefixAndTailTest
required_spec313_cancelMustMakeThePublisherEventuallyDropAllReferencesToTheSubscriber
The tck subscriber was still referenced. Profiling revealed that the
root cause was the VirtualPublisher that holds a reference to the
realPublisher, which was MultiStreamOutputProcessor$SubstreamOutput,
which had the reference to the subscriber. The VirtualPublisher
is created by the Sink.publisher in the test, and the test holds
on to that VirtualPublisher reference.

The solution is to null out realPublisher field in the VirtualPublisher.

The old workaround with the NullSubscriber was removed.

Also made Sink.publisher reject additional subscribers.

akka-stream/src/main/scala/akka/stream/impl/StreamOfStreamProcessors.scala

@@ -68,7 +68,6 @@ private[akka] object MultiStreamOutputProcessor {
     override def cancel(): Unit = {
       if (!downstreamCompleted) {
         closePublisher(Cancelled)
-        subscriber = NullSubscriber // FIXME  unreference real subscriber, should not be needed after #16986
         downstreamCompleted = true
       }
     }