remove RelativePanel

This would be included in WinUI 3.0, which MS plans to make open source.
[LINK](https://github.com/Microsoft/microsoft-ui-xaml/issues/36#issuecomment-492910592)

src/Shared/HandyControlDemo_Shared/Data/MessageToken.tt

@@ -51,8 +51,7 @@ var controlList = new List<string>
 	"DialogDemoCtl",
 	"SideMenuDemoCtl",
 	"GridDemoCtl",
-	"RangeSliderDemoCtl",
-	"RelativePanelDemoCtl"
+	"RangeSliderDemoCtl"
 };
 var styleList = new List<string>
 {