Do not use CredentialCashe when building for uap.

* HttpClientHandler for uap does not support a non-NetworkCredentialManager type.
* #if/#endif ...ing the piece of code that is causing this PNSE error.
* Regression was introduced via: dotnet#2668

src/System.Private.ServiceModel/src/System/ServiceModel/Channels/HttpChannelFactory.cs

@@ -366,9 +366,11 @@ internal async Task<HttpClient> GetHttpClientAsync(EndpointAddress to, Uri via,
                 }
                 else
                 {
+#if !FEATURE_NETNATIVE // HttpClientHandler for uap does not support a non-NetworkCredential type.
                     CredentialCache credentials = new CredentialCache();
                     credentials.Add(GetCredentialCacheUriPrefix(via),
                         AuthenticationSchemesHelper.ToString(_authenticationScheme), credential);
+#endif
                     clientHandler.Credentials = credentials;
                 }