BELFAST, Northern Ireland (Reuters) -- The United States has demanded that the IRA disband after the guerrilla group's astonishing offer to shoot the killers of a murdered Northern Ireland Catholic man. "It's time for the IRA to go out of business," U.S. special envoy Mitchell Reiss said Wednesday. For the IRA's political ally Sinn Fein, Northern Ireland's biggest Irish nationalist party, the U.S. demand was yet another blow to its democratic credentials. Reiss told BBC radio: "It's time for Sinn Fein to be able to say explicitly, without ambiguity, without ambivalence, that criminality will not be tolerated. "You can't sign...