Fix dropdown destroy method

Fix dropdown destroy method

This fixes issue #9339. When destroying a dropdown menu, the dropdown
classes wouldn’t be removed if using a different class structure (for
instance, using custom class names instead of `.menu`). Previous
versions selected all child elements of the menu but on the latest
version only direct `li` childs or  elements with `.menu` class

This commit ensures `ul` elements also get the classes removed, for
those cases where custom classes are used.