[Revan]

You cannot remove that line.
The only thing you can do is put all the Warning Options under a different category, which would serve no purpose.
EDIT: Actually, it would be possible through hacking the admincp/options.php file, but I must say I am highly against such an edit. It is 100% unneeded and adding more hassle to upgrading is not something anyone needs

As for the template delete, you have been smart enough to name all your templates with a prefix of warn_, so you can just place this somewhere:
$DB_site->query("DELETE FROM `" . TABLE_PREFIX . "template` WHERE `title` LIKE 'warn%' ");
(SELECT *
FROM `template`
WHERE title LIKE 'warn%' to test it )