The new fifth-generation iPod touch lacks a light sensor to enable an auto-brightness setting, and its thin body is the reason, according to an email from Apple marketing chief Phil Schiller. Published by 9to5Mac, Schiller’s email was sent in response to a customer who inquired about the lack of an auto-brightness setting, an omission noted in iLounge’s review of the new iPod touch last week. “The 5th generation iPod touch does not…