[iOS] ScrollViewRenderer: clear the renderer (#4489)

On Dispose(), the Renderer is supposed to be cleared on the element.
That's taken care of by VisualElementRenderer`1.Dispose().
Unfortunately, ScrollViewRenderer doesn't inherit from it.

I find it strange that this never was triggered before the Shell, but
I'm learning to live surrounded by strange behaviors.

-fixes #4381

Xamarin.Forms.Platform.iOS/Renderers/ScrollViewRenderer.cs

@@ -160,6 +160,7 @@ namespace Xamarin.Forms.Platform.iOS
 				if (_packager == null)
 					return;
 
+				Element?.ClearValue(Platform.RendererProperty);
 				SetElement(null);
 
 				_packager.Dispose();