Remove usages of \Drupal::url()

1. PHP Parse error: syntax error, unexpected ')' in modules/feeds/src/EventSubscriber/PubSubHubbub.php on line 169
2. The Url class used in PubSubHubbub.php is "GuzzleHttp\Url", not "\Drupal\Core\Url". "\Drupal\Core\Url" should be imported at the top of the file using the "use" keyword. Since that will result into a name collision, one of the two referenced classes should be aliased using the "as" keyword like in the following example:
   

   use My\Full\Classname as Another;
   

   See also http://php.net/manual/en/language.namespaces.importing.php

Looks good to me. Committed.