Remove list custom attributes deprecation

[rmccar]

In #3333 to avoid breaking changes some temporary code needed to put in to continue allowing custom attributes to be set on links within list items. We need to make the changes so that list item custom attributes are now always set on the list item element rather than the link if there is a link in the list item and on the list item if there is no link. This means the functionality is more consistent. On lists where there is only one item the attribute is set on the paragraph element along with any list attributes.