DETROIT — The Pistons continue to ready themselves for the start of free agency. The team is planning to buy out the contract of guard Kemba Walker, who was acquired last week from the New York Knicks, a league source confirmed to The Detroit News. Once the buyout agreement is completed, Walker will become an unrestricted free agent, and he could join another team. Walker, a four-time All-Star, was set to make $9.2 million next season, the final year of his contract. He’ll join his fourth team in the past five seasons, after earning All-Star honors in his final three years in Charlotte, along …