Setting 'smtphosts' is defined in plugin message_email but is used in core

At some point, the smtphosts setting has been moved from the core config into the message_email plugin settings, but the get_mailer() function is still using it regardless of whether the plugin is installed. This doesn't seem like a good idea, as it means a Moodle service could be sending stuff to an SMTP server that the administrator is unable to see or change (apart from forcing it in config.php). To reproduce on a clean Moodle install:

1. Set up an SMTP server in the smtphosts setting
2. Uninstall the message_email plugin
3. Confirm that mail is still being sent to the SMTP server
4. Confirm that the smtphosts setting is unavailable for editing