Remove System.Net.Requests dependency from System.Private.Xml (dotnet/corefx#41111)

Remove System.Net.Requests dependency from System.Private.Xml (dotnet/corefx#41111)

* Remove System.Net.Requests dependency from System.Private.Xml

Anything that uses System.Xml ends up implicitly referencing this .dll, which in a trimmed default MVC app is 97K.  The only thing it's used for is as part of XmlResolver to download the specified url.  We can instead remove the usage of WebRequest.Create and replace it with usage of HttpClient, which is already brought in because System.Net.Requests uses it to implement HttpWebRequest.

* Address PR feedback

And other minor cleanup.  Also fix uap build.

Commit migrated from https://github.com/dotnet/corefx/commit/f873a33056ab318c02d295f816065a3d2ad3736b