Alan Khazei, Elizabeth Warren’s biggest rival for the Democratic US Senate nomination, said through a spokesman yesterday that he planned to drop out of the race, all but clearing the field for a direct challenge from Warren to Republican US Senator Scott Brown. Khazei, co-founder of the national service program CityYear, is the third Democrat to leave the 2012 Senate election since Warren announced her candidacy last month, with backing from national Democratic insiders and a wave of progressive enthusiasm.