Over 3,000 low-level marijuana cases were thrown out Wednesday as Manhattan’s top prosecutor furthered a shift away from arresting and prosecuting many people for small-time pot offenses in the nation’s biggest city. Misdemeanor and violation-level pot possession cases that had sat open for as long as 40 years were dismissed in a matter of minutes after Manhattan District Attorney Cyrus R. Vance Jr. asked a court to scrap 3,042 warrants for people who missed court dates and to toss out the cases themselves