Amazon-owned Withoutabox, the dominant indie film festival submission platform for the last decade, has decided to end its service. In a letter to users, the service – run through IMDbPro – said it would “phase out” the service over the next year. Withoutabox was the official platform for Sundance and had been used by the LA Film Festival, the Toronto Film Festival, and hundreds of smaller events, with the platform claiming it processed more than 100,000…