8 – Registration email showing default mail message as well new custom message

How to stop default mail , i want to override default mail with my custom body message?

Here is my code :

function mag_api_mail_alter(&$message) {
  // only alter register forms
  if (!empty($message('id')) && $message('id') == 'user_register_no_approval_required') {

  // make it a bit neater to access fields
    $nids = Drupal::entityQuery('node')->condition('type','email_template')->execute();
    $nodes =  DrupalnodeEntityNode::loadMultiple($nids);
    $data_fields = array();
    foreach ($nodes as $new_data){
        $data_fields() = $new_data->get('body')->getValue();
    $contact_message = $message('params')('account');
    $first_name = $contact_message->field_first_name->value;
    $username_email = $contact_message->get('name')->value;
    $pass = 'test';
    $app_url = 'abcd.com';
    /*custom field added through configuration*/
    $to_replace_keys = array("(user_name)","(user_email)","(user_password)","(app_url)");
    $replace_keys =array($first_name,$username_email,$pass,$app_url);
    $success_email_msg = str_replace($to_replace_keys, $replace_keys,$data_fields(0)(0)('value'));
    // Assigning new message to mail body.
    $message('body')() = $success_email_msg;

Its is working but i am not able to find out why is this showing default mail message as well new body message.

I just want to show my custom body message.