Wordpress - 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;
}

Thanks for all your help, i finally resolved it. I created a functions.php for my child theme, then add

// we get the instance of the class
$instance = WPSEO_Frontend::get_instance();
/* then we remove the function
    You can remove also others functions, BUT remember that when you remove an action or a filter, arguments MUST MATCH with the add_action
    In our case, we had :
    add_action( 'wpseo_head', array( $this, 'debug_marker' ), 2 );

    so we do : 
    remove_action( 'wpseo_head', array( $this, 'debug_marker' ), 2 );
    */
    remove_action( 'wpseo_head', array( $instance, 'debug_marker' ), 2 );

I don't think you are going to be able to do that using remove_action. The function argument in remove_action will not help you as the debug_marker() function was not the function that was used in the add_action() call.

Yoast presumably has something like add_action( "wp_head", "head" ) in his code. So you can remove the "head" function, but debug_marker was not explicitly added as an action.

You could

1. Edit Yoast's source file and remove the debug comment line.
2. Extend the WPSEO_Frontend class and overload the debug_marker function to return "". TBH, I'm not sure how this would work in terms of WP loading the plugin, but could be worth investigating.