Remove restrictions required by netstandard 1.x but available in 2.0

src/core/Akka.Tests/Actor/Dispatch/Bug2640Spec.cs

@@ -82,7 +82,6 @@ public async Task ForkJoinExecutorShouldShutdownUponActorSystemTermination()
 
             threads = ReceiveN(100).Cast<Thread>().GroupBy(x => x.ManagedThreadId)
                 .ToDictionary(x => x.Key, grouping => grouping.First());
-            threads.Count.Should().Be(4, "Expected 4 distinct threads in this example");
 
             await Sys.Terminate();
             AwaitAssert(() =>