The maniac accused of pushing a 76-year-old man down a flight of Manhattan subway stairs to his death appeared in an NYC court just hours after the heinous attack — and strolled out, because no one realized he was the killer, The Post has learned. Rhamell Burke acted crazy when he showed up Friday morning at Manhattan Criminal Court for a hearing on a third-degree assault case, sources and records showed. “He was acting like a nut all morning,” a law enforcement source said. “Even if they didn’