Remove unused CSS

These styles are not used since we killed the HMRC manuals support so remove
Remove redundant class

Since all manual-subsections are collapsible because we removed the hmrc multi-tier manuals, remove all manual-subsection instances and just use js-openable instead

This commit also removes print styles for 'linked-title' which is an HMRC style we are no longer use
Merge pull request #95 from alphagov/unify-shared-styles

Unify shared styles