Remove redundant css and javascript

DO NOT MERGE until the following pull requests have been merged —

alphagov/frontend/pull/512
alphagov/smart-answers/pull/700
alphagov/licence-finder/pull/51
alphagov/calendars/pull/57
alphagov/prototyping-v2/pull/1
alphagov/transaction-wrappers/pull/42

This pull request removes css and javascript that is only used in conjunction with the 'Not what you're looking for' link seen on small viewports. The 'Not what you're looking for' link is to be deleted as shown in the pull requests above.