Revert ThreadPool.SetMinThreads(0,0) (#5059)

Based on user feedback on #4983

Author: Aaronontheweb

src/core/Akka/Actor/ActorSystem.cs

@@ -278,10 +278,6 @@ public static ActorSystem Create(string name)
 
         private static ActorSystem CreateAndStartSystem(string name, Config withFallback, ActorSystemSetup setup)
         {
-            // allows the ThreadPool to scale up / down dynamically
-            // by removing minimum thread count, which in our benchmarks
-            // appears to negatively impact performance
-            ThreadPool.SetMinThreads(0, 0); 
             var system = new ActorSystemImpl(name, withFallback, setup, Option<Props>.None);
             system.Start();
             return system;