function ag_remove_cmb_metaboxes(){
        global $post;
    $id = $post->ID;
if( $id == 21){?>
    <script type="text/javascript">
    jQuery(document).ready(function($){
    jQuery('#test_metabox_img') .hide();
    });
    </script>
<?php }

}
add_action( 'admin_head', 'ag_remove_cmb_metaboxes' );