Remove PMPro Series filter for posts to use default restricted content messages.

remove-pmpro-series-filter.php

<?php
/**
 * Remove PMPro Series (This post belongs to series X) message. Show default content restriction instead.
 * Follow this guide to add code to your WordPress site: https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_pmpro_remove_series_filter() {
    remove_filter( 'pmpro_non_member_text_filter', 'pmpros_pmpro_text_filter' );
    remove_filter( 'pmpro_not_logged_in_text_filter', 'pmpros_pmpro_text_filter' );
}
add_action( 'init', 'my_pmpro_remove_series_filter' );