Hide Page Template control when a page is the same as the page_for_posts

The control should honor the same condition as in PHP: `get_option( 'page_for_posts' ) != $post->ID`. See https://github.com/xwp/wordpress-develop/blob/4.6.0/src/wp-admin/includes/meta-boxes.php#L820

See todo in https://github.com/xwp/wp-customize-posts/pull/227
