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/HandyControl_Shared/Data/Enum/RelativeStatus.cs

@@ -1,37 +0,0 @@
-namespace HandyControl_Shared.Data.Enum
-{
-    public enum RelativeStatus
-    {
-        LeftOf,
-
-        Above,
-
-        RightOf,
-
-        Below,
-
-        AlignLeftWith,
-
-        AlignTopWith,
-
-        AlignRightWith,
-
-        AlignBottomWith,
-
-        AlignHorizontalCenterWith,
-
-        AlignVerticalCenterWith,
-
-        AlignLeftWithPanel,
-
-        AlignTopWithPanel,
-
-        AlignRightWithPanel,
-
-        AlignBottomWithPanel,
-
-        AlignHorizontalCenterWithPanel,
-
-        AlignVerticalCenterWithPanel
-    }
-}