Remove IsVisible changes from Window/Popup.

These were from a previous attempt to do what #935 does.

src/Avalonia.Controls/Primitives/Popup.cs

@@ -205,7 +205,6 @@ namespace Avalonia.Controls.Primitives
             }
 
             _popupRoot.Position = GetPosition();
-            IsVisible = true;
 
             if (_topLevel == null && PlacementTarget != null)
             {

src/Avalonia.Controls/Window.cs

@@ -242,8 +242,6 @@ namespace Avalonia.Controls
             {
                 PlatformImpl.Show();
             }
-
-            IsVisible = true;
         }
 
         /// <summary>
@@ -293,7 +291,6 @@ namespace Avalonia.Controls
                         result.SetResult((TResult)_dialogResult);
                     });
 
-                IsVisible = true;
                 return result.Task;
             }
         }