get rid of explicit GrpcEnvironment.Shutdown()

src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs

diff --git a/src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs b/src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs
index 024377e..80c35fb 100644
--- a/src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs
+++ b/src/csharp/Grpc.HealthCheck.Tests/HealthClientServerTest.cs
@@ -71,10 +71,9 @@
         [TestFixtureTearDown]
         public void Cleanup()
         {
-            channel.Dispose();
+            channel.ShutdownAsync().Wait();
 
             server.ShutdownAsync().Wait();
-            GrpcEnvironment.Shutdown();
         }
 
         [Test]