Remove the (hidden as an HTML comment) comment that is added to mark a site as using Paid Memberships Pro

remove_pmpro_footer_link.php

/*
  Remove the (hidden) comment that is added to mark a site as using Paid Memberships Pro
  
  Add this code to your active theme's functions.php or a custom plugin.
*/
function remove_pmpro_footer_link() {
  remove_action("wp_footer", "pmpro_footer_link");
}
add_action('init', 'remove_pmpro_footer_link');