Remove an action from an external Class

A simple way to achieve this (but without the Class approach) is by filtering the output of wp_head action hook using the output buffering.

In your theme’s header.php, wrap the wp_head() call with ob_start($cb) and ob_end_flush(); functions like:

ob_start('ad_filter_wp_head_output');
wp_head();
ob_end_flush();

Now in theme functions.php file, declare your output callback function (ad_filter_wp_head_output in this case):

function ad_filter_wp_head_output($output) {
    if (defined('WPSEO_VERSION')) {
        $output = str_ireplace('<!-- This site is optimized with the Yoast WordPress SEO plugin v' . WPSEO_VERSION . ' - http://yoast.com/wordpress/seo/ -->', '', $output);
        $output = str_ireplace('<!-- / Yoast WordPress SEO plugin. -->', '', $output);
    }
    return $output;
}

If you want to do all that through the functions.php without editing header.php file, you can hook to get_header and wp_head action hooks to define the output buffering session:

add_action('get_header', 'ad_ob_start');
add_action('wp_head', 'ad_ob_end_flush', 100);
function ad_ob_start() {
    ob_start('ad_filter_wp_head_output');
}
function ad_ob_end_flush() {
    ob_end_flush();
}
function ad_filter_wp_head_output($output) {
    if (defined('WPSEO_VERSION')) {
        $output = str_ireplace('<!-- This site is optimized with the Yoast WordPress SEO plugin v' . WPSEO_VERSION . ' - http://yoast.com/wordpress/seo/ -->', '', $output);
        $output = str_ireplace('<!-- / Yoast WordPress SEO plugin. -->', '', $output);
    }
    return $output;
}