Remove assets group as that isn't needed in Rails 4.

I added this based on my experience in MyFDB using Rails 3.2 and I
figured it was good to have since the Rails 4 generator doesn't add it
but the upgrade guides state you should remove it:
http://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading
-from-rails-3-2-to-rails-4-0