get rid of explicit GrpcEnvironment.Shutdown()

src/csharp/Grpc.Core.Tests/ChannelTest.cs

src/csharp/Grpc.Core.Tests/ClientServerTest.cs

src/csharp/Grpc.Core.Tests/CompressionTest.cs

src/csharp/Grpc.Core.Tests/ContextPropagationTest.cs

src/csharp/Grpc.Core.Tests/Grpc.Core.Tests.csproj

src/csharp/Grpc.Core.Tests/GrpcEnvironmentTest.cs

src/csharp/Grpc.Core.Tests/ResponseHeadersTest.cs

src/csharp/Grpc.Core.Tests/ServerTest.cs

src/csharp/Grpc.Core.Tests/ShutdownTest.cs

src/csharp/Grpc.Core.Tests/TimeoutsTest.cs

src/csharp/Grpc.Core/Channel.cs

src/csharp/Grpc.Core/GrpcEnvironment.cs

src/csharp/Grpc.Core/Internal/AsyncCall.cs

src/csharp/Grpc.Core/Internal/AsyncCallBase.cs

src/csharp/Grpc.Core/Internal/AsyncCallServer.cs

src/csharp/Grpc.Core/Internal/BatchContextSafeHandle.cs

src/csharp/Grpc.Core/Internal/ChannelSafeHandle.cs

src/csharp/Grpc.Core/Internal/DebugStats.cs

src/csharp/Grpc.Core/Internal/GrpcThreadPool.cs

src/csharp/Grpc.Core/Internal/ServerCallHandler.cs