After more than 27 years, Herrell’s Ice Cream shop in Harvard Square is shutting its doors, just months after another location closed in Allston. Jeffrey Stanett, owner of the Dunster Street spot known for premium sweet treats, said the business had been losing money for years and he could no longer afford to subsidize the store in the face of higher rents, increased franchise fees, and growing competition from rivals like J.P. Licks.