RIP, Tab.After nearly 60 years, Coca-Cola is discontinuing its first-ever diet soda, Tab, which acquired a huge fan base in the ’70s and ’80s and maintained a small but devoted following through the years.”Tab had an amazing run,” said Robert Bixby, executive director of the Concord Coalition and a longtime Tab enthusiast. Bixby said messages of condolences have been pouring in. “As a business decision