Removing UWP support

[turanszkij]

UWP platform will no longer be maintained because:

• Wicked Engine was removed from Microsoft Store because of Microsoft's policy changes
• UWP file handling was slow compared to other platforms
• Xbox UWP was very slow, Xbox native build is preferred
• UWP required certain threading constraints that other platforms didn't
• UWP development requires Nuget package management
• UWP often broke automated nightly builds for unknown reasons