Fix "go back" link in ballot page

Since the `@ballot_referer` variable was only set in the lines
controller, it didn't work when we accessed the ballot page without
adding a line.

Note it still doesn't work if we access the ballot page directly by
entering the URL in the browser's address bar.

app/controllers/budgets/ballot/lines_controller.rb

@@ -7,7 +7,6 @@ module Budgets
       before_action :load_tag_cloud
       before_action :load_categories
       before_action :load_investments
-      before_action :load_ballot_referer
 
       authorize_resource :budget
       authorize_resource :ballot
@@ -67,10 +66,6 @@ module Budgets
           @categories = Tag.category.order(:name)
         end
 
-        def load_ballot_referer
-          @ballot_referer = session[:ballot_referer]
-        end
-
         def load_map
           @investments ||= []
           @investments_map_coordinates = MapLocation.where(investment: @investments).map(&:json_data)