Wordpress uses PHP mail() by default. If you disable/uninstall any extensions that try to manage email sending and delete the configuration lines above from wp-config.php, it should work fine.

You can check if PHP’s mail() function is working properly by creating a file with the following content:

<?php
    ini_set('display_errors', 1);
    error_reporting(-1);
    mail ('you@example.com', 'Postfix Test', 'A test email') || print_r(error_get_last());
?>

Place it in your document root (e.g. /var/www/html) and browse to it. If it’s blank, everything went well and you should receive an email shortly.

PHP uses an MTA to send it’s mails trough mail(),
if you want to override it on system level you could edit /etc/postfix/main.cf and add

relayhost = [smtp.domain.tld]:587

if you are on an older system, you might need to configure sendmail instead