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.cs

@@ -98,8 +98,6 @@
 	
         public static readonly string RangeSliderDemoCtl = nameof(RangeSliderDemoCtl);
 	
-        public static readonly string RelativePanelDemoCtl = nameof(RelativePanelDemoCtl);
-	
         public static readonly string ButtonDemoCtl = nameof(ButtonDemoCtl);
 
         public static readonly string RepeatButtonDemoCtl = nameof(RepeatButtonDemoCtl);

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>
 {

src/Shared/HandyControlDemo_Shared/UserControl/Main/LeftMainContent.xaml

@@ -325,11 +325,6 @@
                                 <Image Source="../../Resources/Img/LeftMainContent/Panel_16x.png"/>
                             </hc:EdgeElement.LeftContent>
                         </ListBoxItem>
-                        <ListBoxItem Style="{StaticResource ListBoxItemNew}" Tag="{x:Static data:MessageToken.RelativePanelDemoCtl}" Content="{x:Static langs:Lang.RelativePanel}">
-                            <hc:EdgeElement.LeftContent>
-                                <Image Source="../../Resources/Img/LeftMainContent/PaneLayout_16x.png"/>
-                            </hc:EdgeElement.LeftContent>
-                        </ListBoxItem>
                         <ListBoxItem Tag="{x:Static data:MessageToken.CoverViewDemoCtl}" Content="{x:Static langs:Lang.CoverView}">
                             <hc:EdgeElement.LeftContent>
                                 <Image Source="../../Resources/Img/LeftMainContent/DetailDataView_16x.png"/>