Remove NullableBool MarkupExtension #3198
Description
Describe the problem this feature would solve
This is now supported in the latest SDKs for all three values natively in XAML. We no longer need to have an extension to provide Null values, we can just use {x:Null}. I'm not sure which SDK this was introduced in, so let's put this in the WinUI 3 bucket, and we can quickly test that it still works there.
I've added System Unit tests testing this behavior to our NullableBool markup extension unit tests for now to aid in that transition.