NEW YORK, March 31 (Reuters) - A U.S. judge on Wednesday dismissed litigation accusing 10 large banks of conspiring to suppress competition in the multi-trillion dollar market for U.S. Treasury securities. The decision by U.S. District Judge Paul Gardephe in Manhattan came in long-running litigation by 21 pension, retirement and benefit funds, as well as unions, banks, individuals, and companies that traded in Treasuries.