A Nicaraguan diplomat was found dead in his Bronx apartment with his throat slashed yesterday when his driver came to pick him up for the UN General Assembly meeting, police sources said. The body of Cesar Mercado, 34, was found shortly after 10:30 a.m. when his driver opened the door of his Grand Concourse apartment and saw him on the floor in a pool of blood, cops said. Investigators are probing the possibility that the killing was caused by a gay relationship gone wrong, police sources said. The sound of men arguing was heard in the apartment before the murder,...