Don't reset available space on Arrange.

This fixes the failing `Should_Add_Containers_For_Items_After_Clear`
however I'm unsure if it's the correct fix. May need to revisit if it
seems to be causing other problems.

src/Avalonia.Controls/VirtualizingStackPanel.cs

@@ -74,7 +74,6 @@ namespace Avalonia.Controls
 
         protected override Size ArrangeOverride(Size finalSize)
         {
-            _availableSpace = finalSize;
             _canBeRemoved = 0;
             _takenSpace = 0;
             _averageItemSize = 0;