OneChicago, the only U.S. exchange for futures pegged to single stocks, is closing its doors.The firm will discontinue operations on Sept. 21 with a final trading date of Sept. 18, when 99.4% of its contracts with open interest are set to expire. Its staff is working with clearing partners and brokers to unwind the other 0.6% of yet-to-be-settled contracts before that date.OneChicago already delisted all products without open interest on Aug....