Clearing cookie on logout and session expiration

Try setting $experation to a negative integer:

function myplugin_cookie_expiration( $expiration, $user_id, $remember ) {
    return $remember ? $expiration : -600;
}
add_filter( 'auth_cookie_expiration', 'myplugin_cookie_expiration', 99, 3 );

From the w3schools PHP page on cookies:

<?php
// set the expiration date to one hour ago
setcookie("user", "", time()-3600);
?>