WASHINGTON — For the first time, a major U.S. contractor has dropped out of the multibillion-dollar effort to rebuild Iraq, raising new worries about the country's growing violence and its effect on reconstruction. Contrack International Inc., the leader of a partnership that won one of 12 major reconstruction contracts awarded this year, cited skyrocketing security costs in reaching a decision with the U.S. government last month to terminate work in Iraq. "We reached a point where our costs were getting to be prohibitive," said Karim Camel-Toueg, president of Arlington, Va.-based Contrack, which had won a $325-million award to rebuild Iraq's...