Registration email showing default mail message as well new custom message
$message['body'][] = $success_email_msg; is appending your variable to the existing array. You'll need to remove the default content from the $message['body'] array if you don't want it
e.g. unset($message['body']['whateverkey']) to remove a specific part. Or $message['body'] = [$success_email_msg]; to reinitialise the variable as a new array