+str #24229 back to Future[] API for stream refs

nitpicks

akka-stream/src/main/scala/akka/stream/javadsl/Source.scala

@@ -26,7 +26,6 @@ import java.util.concurrent.CompletionStage
 import java.util.concurrent.CompletableFuture
 
 import akka.annotation.InternalApi
-import akka.stream.scaladsl.Sink
 
 import scala.compat.java8.FutureConverters._
 
@@ -446,16 +445,6 @@ object Source {
       (s: S) ⇒ read.apply(s).toScala.map(_.asScala)(akka.dispatch.ExecutionContexts.sameThreadExecutionContext),
       (s: S) ⇒ close.apply(s).toScala))
 
-  /**
-   * A local [[Sink]] which materializes a [[SourceRef]] which can be used by other streams (including remote ones),
-   * to consume data from this local stream, as if they were attached in the spot of the local Sink directly.
-   *
-   * Adheres to [[StreamRefAttributes]].
-   *
-   * See more detailed documentation on [[SinkRef]].
-   */
-  def sinkRef[T](): javadsl.Source[T, SinkRef[T]] =
-    scaladsl.Source.sinkRef[T]().asJava
 }
 
 /**
@@ -485,13 +474,6 @@ final class Source[+Out, +Mat](delegate: scaladsl.Source[Out, Mat]) extends Grap
   def mapMaterializedValue[Mat2](f: function.Function[Mat, Mat2]): Source[Out, Mat2] =
     new Source(delegate.mapMaterializedValue(f.apply _))
 
-  /**
-   * INTERNAL API: Unsafe BLOCKING flattening if current materialized value is a Future.
-   */
-  @InternalApi
-  private[akka] def flattenMaterializedValue[Mat2](timeout: FiniteDuration): Source[Out, Mat2] =
-    new Source(delegate.flattenMaterializedValue[Mat2](timeout))
-
   /**
    * Transform this [[Source]] by appending the given processing stages.
    * {{{