Chicago Law Department attorneys tasked with playing legal offense on the city’s behalf say their mission remains the same regardless of who takes the helm of City Hall next spring.The recently launched Affirmative Litigation Division plans to stick around even as Mayor Rahm Emanuel’s term ends in May, ushering new leadership in the Law Department.Corporation Counsel Edward N. Siskel last month told the Chicago City Council he will stay on until a new mayor takes office and appoints his replacement.“As …