Remove unused string

!493 (merged) (against master) used this string, and in !945 (closed) there was a translation added. !493 (merged) didn't make it to the master branch, and I don't think it should be backported because I suppose it would conflict with the GPDR

interface/web/dashboard/lib/lang/it.lng

 <?php
 $wb['welcome_user_txt'] = 'Buongiorno %s';
 $wb['available_modules_txt'] = 'Moduli disponibili';
-$wb['last_login_ip'] = "Ultimo login %s da %s";
 ?>