Remove custom message

You’d want to add this to your functions.php file in your theme or create a new plugin to accomplish this.

/**
 * Modify the default notification message the user will see
 * after registering for an account.
 *
 * @param $message The default message.
 * @return string the updated message.
 */
function my_custom_registration_message( $message ) {
    // here you can modify part of the message or
    // completely override it.
    $message = '';

    return $message;
}

// modify the default registration message
add_filter( 'nua_user_approve_registration_message_default', 'my_custom_registration_message' );

Good luck. Let me know how it goes.