Turn off the new user email notification for user.

index.php

<?php 
function ab610_wp_new_user_notification_email( $wp_new_user_notification ) {
	$wp_new_user_notification['to'] = '';
	return $wp_new_user_notification;
}

add_filter( 'wp_new_user_notification_email', 'ab610_wp_new_user_notification_email' );

// We can use `wp_new_user_notification_email_admin` filter for turn off the admin notification.