Remove the heading supporting documents

The heading is confusing as the following documents aren't always
'supporting'. There is a new publications model in the works that will
let editors better model publications but for now we should remove the
title.

https://www.pivotaltracker.com/story/show/48729367
Merge pull request #510 from alphagov/remove-supporting-docs

Remove the heading supporting documents

https://www.pivotaltracker.com/story/show/48729367