Revert akkadotnet#7497

Author: Arkatufus

src/core/Akka.API.Tests/verify/CoreAPISpec.ApproveStreams.DotNet.verified.txt

@@ -4128,10 +4128,6 @@ namespace Akka.Streams.Implementation.Fusing
         protected override Akka.Streams.Stage.GraphStageLogic CreateLogic(Akka.Streams.Attributes inheritedAttributes) { }
         public override string ToString() { }
     }
-    public class DownstreamCompletedWithNoCauseException : System.Exception
-    {
-        public DownstreamCompletedWithNoCauseException() { }
-    }
     [Akka.Annotations.InternalApiAttribute()]
     public sealed class Expand<TIn, TOut> : Akka.Streams.Stage.GraphStage<Akka.Streams.FlowShape<TIn, TOut>>
     {

src/core/Akka.Streams/Implementation/Fusing/DownstreamCompletedWithNoCauseException.cs

@@ -1297,25 +1297,10 @@ public void InternalOnDownstreamFinish(Exception cause)
         {
             try
             {
+                if (cause == null)
+                    throw new ArgumentException("Cancellation cause must not be null", nameof(cause));
                 if (_lastCancellationCause != null)
                     throw new ArgumentException("OnDownstreamFinish must not be called recursively", nameof(cause));
-                
-                // Some stages might propagate null exceptions due to improper Task continuation handling
-                // (see https://github.com/akkadotnet/Akka.Persistence.Sql/issues/498)
-                // This is a stop gap solution to make sure that Akka.Streams doesn't behave improperly
-                // until we can fix all of those
-                if (cause is null)
-                {
-                    try
-                    {
-                        throw new DownstreamCompletedWithNoCauseException();
-                    }
-                    catch (DownstreamCompletedWithNoCauseException e)
-                    {
-                        cause = e;
-                    }
-                }
-                
                 _lastCancellationCause = cause;
                 CancelStage(_lastCancellationCause);
             }