fix: Error CS0105 The using directive for 'Windows.UI.Xaml.Controls' appeared previously in this namespace Uno.UI (xamarinmac20)

Author: workgroupengineering

src/Uno.UI/UI/Xaml/FrameworkTemplatePool.cs

@@ -36,7 +36,6 @@
 using ViewGroup = AppKit.NSView;
 using Color = AppKit.NSColor;
 using Font = AppKit.NSFont;
-using Windows.UI.Xaml.Controls;
 using DependencyObject = System.Object;
 using AppKit;
 #elif METRO