Improve domain discard

- Extract rake task domain:discard
- Remove background job when keeping a domain

#790

config/schedule.rb

@@ -61,6 +61,10 @@ if @cron_group == 'registry'
   every :day, at: '19:00pm' do
     runner 'Directo.send_receipts'
   end if @environment == 'production'
+
+  every 42.minutes do
+    rake 'domain:discard'
+  end
 end
 
 every 10.minutes do