Windows 10 Version 1903 - June 2021 Samples Update

* DeviceEnumerationAndPairing: Remove unused C++/WinRT code that didn't work anyway.
* FilePicker: Add some comments about how to access the picked file.
* Simple3DGameDX: Improve standard conformance in C++/WinRT sample.
* SocketActivityStreamSocket: Mention CheckNetIsolationTool.exe in the README.
* Geolocation, UserInfo: Ported to C++/WinRT.