Fix mailkey for status mailer notification messages

Looking closer at the drupal_mail() docs, we don't need to prefix our mailkey with the module name, since that now happens automatically in D6 since the module is passed as a separate argument. Furthermore, our hook_mail() implementation is only invoked on our own messages, so we don't have to test the mailkey in there to see if it's one of ours.

Trivial patch, just wanted to open an issue to inform others.