Remove hmrc manuals collapsible support

HMRC manuals are going to be handled by a different bit of this app, and won't be collapsible anymore. This PR removes a lot of the support of HMRC's manuals, which were complicating the codebase and a pain to support. This will allow new features to (https://trello.com/c/opQFZlon/439-manuals-back-button-doesn-t-work-because-of-the-scroll-position-when-sections-are-collasped, https://trello.com/c/IVx0OoUI/214-manuals-grouping-headings-shouldn-t-be-collapsable-3 and https://trello.com/c/S8Kq0FZI/428-manuals-let-editors-choose-which-headings-are-collapsable) be added more easily.