Reinstate 'How government works' in government

This was mistakenly removed in 7460c6709c28696639ab11f96dfa793523ac1363

This is the place where that link gets into the government index.
Remove "UK Presidency of G8 2013" from mainstream

Thanks to the top 3 results feature, this link is now exposed by default, so
we don't need to add it to the mainstream index as a workaround.
Remove now-empty file