Remove the Ninja Forms required fields string above a form with 1+ required strings.

remove-nf-required-fields-string.php

<?php
/**
 * Change text strings
 *
 * @link http://codex.wordpress.org/Plugin_API/Filter_Reference/gettext
 */
function my_nf_required_fields_strings( $translated_text, $text, $domain ) {
	switch ( $translated_text ) {
		case 'Fields marked with a * are required' :
			$translated_text = __( '', 'ninja-forms' );
			break;
	}
	return $translated_text;
}
add_filter( 'gettext', 'my_nf_required_fields_strings', 20, 3 );