BENGHAZI, Libya, Sept 22 (Reuters) - An Islamist militia was driven out of the city of Benghazi early on Saturday in a surge of anger against the armed groups that control large parts of Libya more than a year after the overthrow of Muammar Gaddafi. A spokesman for Ansar al-Sharia, which some U.S. and Libya officials blame for the attack on the U.S. consulate in Benghazi last week in which the U.S. ambassador and three other Americans were killed, said it had evacuated its bases "to preserve security in the city". In a dramatic sign of Libya's fragility, after sweeping...