Issue #3207748 by rwanth: Use Mailchimp Transactional API instead of Mandrill

* Fixed Library check, wasn't firing at all, still referenced old mandrill library
* Removed mailsystem check, which should be left to Drupal core to handle via info.yml dependency
* phpcs cleanup
* Status Report now-unsupported Mandrill module with a link to new Mailchimp Transactional module