don't show receiveSignal from ExtensibleBehavior in Receive

The intended API is Behaviors.receiveMessage(...).receiveSignal { case ... }
but in code completion of the Receive two receiveSignal shows up, the one
from ExtensibleBehavior is confusing.

Removing sealed form Behavior isn't so nice, but what else can we do?

akka-actor-typed/src/main/scala/akka/actor/typed/Behavior.scala

@@ -36,7 +36,7 @@ import akka.actor.typed.scaladsl.{ ActorContext ⇒ SAC }
  */
 @ApiMayChange
 @DoNotInherit
-sealed abstract class Behavior[T] { behavior ⇒
+abstract class Behavior[T] { behavior ⇒
   /**
    * Narrow the type of this Behavior, which is always a safe operation. This
    * method is necessary to implement the contravariant nature of Behavior

akka-actor-typed/src/main/scala/akka/actor/typed/scaladsl/Behaviors.scala

@@ -253,7 +253,7 @@ object Behaviors {
    * signal reception behavior. It's returned by for example [[Behaviors.receiveMessage]].
    */
   @DoNotInherit
-  trait Receive[T] extends ExtensibleBehavior[T] {
+  trait Receive[T] extends Behavior[T] {
     def receiveSignal(onSignal: PartialFunction[(ActorContext[T], Signal), Behavior[T]]): Behavior[T]
   }