Mayor Mitch Landrieu of New Orleans proudly announced Monday that the city had closed a “dark” chapter in its history that was written more than a decade ago. The city has worked out settlements, totaling $13.3 million, in three especially egregious police killings as well as with plaintiffs in other serious cases involving police brutality and lethal use of force. Most of the cases occurred during the time of Hurricane Katrina, in 2005.