Remove underlines from content item links

https://www.pivotaltracker.com/story/show/80192086

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.