George Bratsenis, a 73-year-old ex-Marine with a penchant for violent crime that included drug dealing, armed robbery, and murder, returned to federal court Thursday to plead guilty to the contract killing of a Hudson County political operative more than seven years ago. But his admission brought no further clarity to the still mysterious murder-for-hire of Michael Galdieri that was paid for in cash by Democratic consultant Sean Caddle, who has already pleaded guilty. No new information was revealed during the videoconference pleading before U.S. District Judge John Michael Vazquez in Newark, …