remove old assets before compiling

doc/update/6.0-to-6.1.md

@@ -53,6 +53,7 @@ sudo -u git -H bundle install --without development test mysql --deployment
 sudo -u git -H bundle exec rake db:migrate RAILS_ENV=production
 sudo -u git -H bundle exec rake migrate_iids RAILS_ENV=production
+sudo -u git -H bundle exec rake assets:clean RAILS_ENV=production
 sudo -u git -H bundle exec rake assets:precompile RAILS_ENV=production
 sudo -u git -H bundle exec rake cache:clear RAILS_ENV=production
 ```