How to Hide Contact Form Submissions Meta Boxes

function plt_hide_contact_form_submissions_metaboxes() {
	$screen = get_current_screen();
	if ( !$screen ) {
		return;
	}

	//Hide the "Mail" meta box.
	remove_meta_box('wpcf7s_mail', $screen->id, 'normal');
	//Hide the "Posted" meta box.
	remove_meta_box('wpcf7s_posted', $screen->id, 'normal');
	//Hide the "Overview" meta box.
	remove_meta_box('wpcf7s_actions', $screen->id, 'side');
}

add_action('add_meta_boxes', 'plt_hide_contact_form_submissions_metaboxes', 20);

Where do I put this code?