Remove old promoted links code

This is replaced by the new best-bets implementation.  Best bets for
"jobs" and "visa" are populated on production in the new system, and we
want to retire the promoted link for "budget 2014", so the new
implementation covers all the result adjustments made by this code.

This implements https://www.pivotaltracker.com/story/show/71295084
Merge pull request #237 from alphagov/remove-promoted-links

Remove old promoted links code

https://www.pivotaltracker.com/story/show/71295084
Remove old healthcheck code

Remove support for scraping HTML results pages, and for accessing the
"combined" search endpoint.