Remove 'mailbox_mail' variable

Per #2113257: Include support for mailcomment and Drupal comments, oa_mailhandler is going to use the 'mailcomment_mailbox' variable rather than 'mailbox_mail' and it will overwrite the 'reply-to' header itself in oa_mailhandler_mail_alter() - so, it won't be necessary for oa_messages_build_notify_options() to mess around with it at all.

I'll attach the patch in a moment!