React to HttpAbstractions namespace changes

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

Author: dougbu

src/Microsoft.AspNetCore.IISPlatformHandler/IISPlatformHandlerMiddleware.cs

@@ -9,7 +9,6 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Http.Features.Authentication;
-using Microsoft.AspNetCore.Http.Features.Authentication.Internal;
 using Microsoft.Extensions.Internal;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;

src/Microsoft.AspNetCore.Server.IISIntegration/IISMiddleware.cs

@@ -9,7 +9,6 @@
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Http.Features.Authentication;
-using Microsoft.AspNetCore.Http.Features.Authentication.Internal;
 using Microsoft.Extensions.Internal;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
@@ -60,7 +59,7 @@ public async Task Invoke(HttpContext httpContext)
                 _logger.LogTrace($"'{MSAspNetCoreToken}' does not match the expected pairing token '{_pairingToken}', skipping {nameof(IISMiddleware)}.");
                 await _next(httpContext);
                 return;
-            }            
+            }
 
             if (_options.ForwardClientCertificate)
             {