React to HttpAbstractions namespace changes

- aspnet/HttpAbstractions#549 and aspnet/HttpAbstractions#592
- clean up `using`s

Author: dougbu

src/Microsoft.AspNetCore.Server.Kestrel.Https/HttpsConnectionFilter.cs

@@ -3,11 +3,9 @@
 
 using System;
 using System.Net.Security;
-using System.Security.Authentication;
 using System.Security.Cryptography.X509Certificates;
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Http.Features;
-using Microsoft.AspNetCore.Http.Features.Internal;
 using Microsoft.AspNetCore.Server.Kestrel.Filter;
 
 namespace Microsoft.AspNetCore.Server.Kestrel.Https

test/Microsoft.AspNetCore.Server.KestrelTests/DummyApplication.cs

@@ -5,7 +5,6 @@
 using System.Threading.Tasks;
 using Microsoft.AspNetCore.Hosting.Server;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.AspNetCore.Http.Features;
 
 namespace Microsoft.AspNetCore.Server.KestrelTests