Remove header/footer toggle props from Section component

The Section component attempts to make its header and footer slots optional by wrapping the Twig blocks in {% if %} conditionals based on boolean props. This doesn't really work; the slots still exist because they're part of the structure of the Twig file, not part of its output. Canvas displays them funkily if they are hidden while empty.

We should just remove those boolean props and let the slots always exist, making sure that they do not print any output if they are left empty. This will break Byte site template default content, so this is postponed until after the approaching public demos are over.