Replacing a core WordPress function
You'll need to name your copy of wp_delete_attachment to a unique name. Perhaps namespace it with your site name like function my_site_wp_delete_attachment().
Also, I believe you'll need to use add_action instead of add_filter.
remove_action( 'delete_attachment', 'wp_delete_attachment' );add_action( 'delete_attachment', 'my_site_wp_delete_attachment');