SPRINGFIELD — State Rep. Michael J. Madigan, a Chicago Democrat who virtually set Illinois’ political agenda as House speaker before he was ousted last month, announced Thursday that he is resigning his seat in the legislature.The 78-year-old Madigan, who was elected speaker in 1983 and served 36 of the next 38 years at the helm, was tarnished by a federal investigation of statehouse bribery announced last summer that implicated him. He has not been charged with wrongdoing and maintains his innocence. But he lost his bid …