Apply suggestions from code review

Author: Tratcher

src/Servers/Kestrel/shared/test/TransportTestHelpers/HostNameIsReachableAttribute.cs

@@ -4,7 +4,6 @@
 using System;
 using System.Net;
 using System.Net.Sockets;
-using System.Runtime.InteropServices;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Testing;