Remove .page class from a page template. Source: https://wordpress.org/support/topic/how-to-delete-page-class-from-body-on-one-page-template?replies=5#post-2395764

remove-class-from-page-template.php

<?php

//* Remove .page class from page template file portfolio.php
add_filter('body_class', 'prefix_remove_body_class', 20, 2);
function prefix_remove_body_class($wp_classes) {
    if (is_page_template('portfolio.php')):
        foreach ($wp_classes as $key => $value) {
            if ($value == 'page') unset($wp_classes[$key]);
        }
    endif;
    return $wp_classes;
}