Remove hard-coded settings used/supported by mailer handler

Support for the previously hard-coded settings is in place. This means mailer.json files containing one or all of smtp_address, smtp_port, smtp_domain, mail_to and mail_from keys will still work.

The new approach is to advocate using keys that are natively supported by Net::SMTP. This allows for much more flexibility and should result in more people being able to use this handler.