delete link to order in ORDER_CREATION_NOTIFICATION_SUBJECT

-- HikaShop version -- : 3.1
-- Joomla version -- : 3.7
-- PHP version -- : 7.1

How?

Hi,

ORDER_CREATION_NOTIFICATION_SUBJECT is the translation for the subject of the email sent to the customer when an order is created. There are no links in the subject of emails, so I don't understand what you mean.
Please provide more context, like for example some screenshot of what you're talking about.

Please see the screenshot. I thought that is this translation string. I would like to keep the order number but not linked.

Hi,

That's probably a translation string yes, but that's not the subject of the email.
That's just some text near the top of the email.
Also, the translation key is:
ORDER_CREATION_SUCCESS_ON_WEBSITE_AT_DATE="We are pleased to confirm the creation of your order No. %s at %s on the %s at %s"
So as you can see, changing the translation won't help you remove the link as the %s of the link also includes the order number.
If you want to remove the link but keep the number, you'll have to edit the email via the menu System>Emails and remove the line:
$url = '<a href="'.$order_url.'">'. $url.'</a>';
in the "preload" of the email.