Remove Big Energy Saving Week text

It's hard to find out when Big Energy Saving Week is. It may have
started on the [27th January 2014][1], or [20th October 2014][2],
or both. And it looks like 2015/16 [hasn't been announced yet][3].

None of those are in December, so remove this text from the
homepage.

[1]: https://www.gov.uk/government/news/big-energy-saving-week-2014
[2]: http://www.citizensadvice.org.uk/index/campaigns/current_campaigns/besw.htm
[3]: http://www.citizensadvice.org.uk/index/campaigns/current_campaigns/besw/bigenergysavingweek.htm
Merge pull request #708 from alphagov/remove-big-energy-saving-week

Remove Big Energy Saving Week text