Invoice notification mails are never removed from the permanent directory

Invoice notification mails are sent with $services.mailsender.send(message), which uses the memory mail listener. With that listener the serialized mail (invoice PDF included) is never removed from <permanent directory>/mails/ after being sent (XWIKI-25742), so the directory keeps growing with one file per invoice mail. These mails also don't appear in the Mail Sending Status administration, and failed ones can't be resent.

The mail should be sent with the database mail listener.