Remove suspicious code fragment in ActorMaterializer.cs (#6216)

Remove duplicate SubscriptionTimeoutSettings field checking and check missing MaxFixedBufferSize field

src/core/Akka.Streams/ActorMaterializer.cs

@@ -623,7 +623,7 @@ public override bool Equals(object obj)
                 s.SyncProcessingLimit == SyncProcessingLimit &&
                 s.IsFuzzingMode == IsFuzzingMode &&
                 s.IsAutoFusing == IsAutoFusing &&
-                s.SubscriptionTimeoutSettings == SubscriptionTimeoutSettings &&
+                s.MaxFixedBufferSize == MaxFixedBufferSize &&
                 s.StreamRefSettings == StreamRefSettings;
         }