Don't bring old parent window to front.

When a window is closed, its parent window is set to null, which caused this code to run. This line caused the parent window to be brought to front, and should have also caused any other child windows to be brought to front, but it seems OSX ignored those requests, causing #8878.

Simply don't bring the parent window to the front when a child window is closed. I couldn't work out why this code was necessary anyway, and all integration tests still pass.

Fixes #8878.

native/Avalonia.Native/src/OSX/WindowImpl.mm

@@ -91,8 +91,6 @@ HRESULT WindowImpl::SetParent(IAvnWindow *parent) {
         if(_parent != nullptr)
         {
             _parent->_children.remove(this);
-            
-            _parent->BringToFront();
         }
 
         auto cparent = dynamic_cast<WindowImpl *>(parent);