I will post here the solution to a problem that i confrunted a few days.
If you want to eliminate [Shop Name] from the Order Confirmation Email Subject, this is the solution for Prestashop 1.7:
classes/Mail.php
Change:
$subject = '['.Configuration::get('PS_SHOP_NAME', null, null, $idShop).'] '.$subject;
To:
//$subject = '['.Configuration::get('PS_SHOP_NAME', null, null, $idShop).'] '.$subject;
Just comment that row.