Remove the code that sends a test email

The configuration form used for the module allows to send a test email, but that email is sent using the email plugin set for the site, which could not be the plugin implemented by this module.

Essentially, there isn't any relation between the success/failure in sending the test email and the configuration values entered in the form. The current submission handler doesn't even show any error in the user interface (as errors is logged), but even if it would be showing an error in the user interface, it won't make clear which configuration value is wrong.

The code to send a test email needs to be removed. That isn't the correct way to validate the configuration values entered in the form.