How to disable new user registered email to admin
This email is not configurable via the UI.

You can use hook_mail_alter(), which will allow you to stop the sending of a message.
So you would create a custom module that does this:

/**
 * Implements hook_mail_alter().
 */
function MODULE_NAME_mail_alter(&amp;$message) {
  // Stop the default drupal email that goes out to admins when a user
  // registers on the site. An alternative email is sent out via other means.
  if ($message['key'] == 'register_pending_approval_admin') {
    $message['send'] = FALSE;
  }
}