A 16-year-old girl was killed and four people were wounded by gunfire Thursday after two groups clashed on a crowded Brooklyn boardwalk, cops and witnesses said. Two of the victims were bystanders, sources said. One man used an umbrella to crack another man in the head as the fight erupted on the Brighton Beach Boardwalk near Brighton Sixth St. about 5:20 p.m. Suddenly, a gunman squeezed off about eight rounds, first shooting in the air before aiming at the brawling crowd, cops and witnesses said. Onlookers scurried into nearby stores and restaurants, huddling in fear as the bullets flew near...