NotificationRecipientSet.remove() does not remove the person email addresses

[Curtis Hovey]

A question email job test on qastaging failed with:
line 85, in getReason
    raise UnknownRecipientError(person)
UnknownRecipientError: <Person at 0xb4f3690 sinzui (Curtis Hovey)>

The code that prepared the subscribers recipient removed the owner:
        elif question_recipient_set == QuestionRecipientSet.SUBSCRIBER:
            recipients = self.question.getRecipients()
            if self.question.owner in recipients:
                recipients.remove(self.question.owner)
            return recipients

But remove() only removed the person from the person to recipient dict;
the person is still in the email address dict. The call to getEmails() uses the
later dict, which returns a user that was removed from the reason set.