Remove unused Campaign model

The only way to use campaigns is to manually insert them in the
database, which IMHO isn't very practical.

We're going to change every piece of code that generates an Ahoy event
and, in this case, the easiest way to change the Campaing model so it
doesn't use Ahoy events is to simply remove it.

Note we're keeping the database tables until we release a new version,
just in case some Consul Democracy installations are using them. We'll
inform in the release notes that we'll remove the campaigns table after
the release, so existing installations using the `campaigns` table can
move the data somewhere else before we remove the table.

app/controllers/application_controller.rb

@@ -11,7 +11,6 @@ class ApplicationController < ActionController::Base
 
   before_action :ensure_signup_complete
   around_action :switch_locale
-  before_action :track_email_campaign
   before_action :set_return_url
 
   check_authorization unless: :devise_controller?
@@ -91,13 +90,6 @@ class ApplicationController < ActionController::Base
       end
     end
 
-    def track_email_campaign
-      if params[:track_id]
-        campaign = Campaign.find_by(track_id: params[:track_id])
-        ahoy.track campaign.name if campaign.present?
-      end
-    end
-
     def set_return_url
       if request.get? && !devise_controller? && is_navigational_format?
         store_location_for(:user, request.fullpath)