The MTA’s top lawyer is abruptly stepping down at the end of July with no successor immediately named — just months after The Post revealed the agency’s legal costs ballooned under her tenure. “After fourteen remarkable years with the Metropolitan Transportation Authority, I have decided that the time has come for me to begin a new chapter,” the email begins, noting that her last day with the agency will be July 31.MTA General Counsel Paige Graves quietly announced her resignation, according to a June 30 email obtained by The Post. MTAGraves’ departure caps a turbulent season for the transit authority’s...