Remove links to the legacy SORN/check tax services

DVLA turned off the old SORN and Check Vehicle Tax services on 30
September 2014 so continuing to link to these could confuse users.
Merge pull request #703 from alphagov/remove-legacy-dvla-services

Remove links to the legacy SORN/Check Vehicle Tax services
Remove unused Sass from start pages

The functionality has been removed in previous commits but the Sass has
been left behind. Remove a collection of Sass which is no longer used by
any pages.
Merge pull request #704 from alphagov/delete-unused-sass

Remove unused Sass from start pages
Tweak the descriptions for topics in search

The previous wording was "Everything on GOV.UK about ..." which was
actually incorrect.  This commit changes it to "List of information
about ..." which is correct, but not great.

Future work will seek to improve this.
Merge pull request #705 from alphagov/tweak-wording-for-subtopics-in-search

Tweak the descriptions for topics in search