Remove underlines from content item links

This removes underlines from links to content items on topic and 
sub-topic pages.

This will keep consistency between the Services and Information page in
Whitehall, where links to content items already omit an underline.

We've only ever tested topic and sub-topic pages without underlines, and
I can't seem to find any discussion or rationale for the underline 
appearing in the first place. 

I now think that this has probably happened by accident, possibly at the
point when we moved specialist sectors out to the collections app.
Merge pull request #45 from alphagov/remove-topic-link-underline

Remove underlines from content item links