WASHINGTON — The Obama administration on Tuesday replaced two top Justice Department officials associated with an ill-fated investigation into a gun-trafficking network in Arizona that has been at the center of a political conflagration. Attorney General Eric H. Holder Jr. announced the resignation of the United States attorney in Phoenix, Dennis K. Burke, and the reassignment of the acting director of the Bureau of Alcohol, Tobacco, Firearms and Explosives, Kenneth E. Melson. The two officials became the highest-profile political casualties yet in the fallout from a disputed effort to take down a weapons-smuggling ring based in Arizona and linked to...