Yes, delete_expired_transients is a cron event that runs once per day and the function delete_expired_transients() is automatically called when the cron event runs — see wp-includes/default-filters.php. So you do not need to call the function manually like you did in your my_custom_fn() function.
And if you use a plugin like WP Crontrol, you can easily view the cron events in your site and the actions (functions) that will be called when a specific cron event runs.