WASHINGTON — Billionaire investor Carl Icahn, who resigned Friday from an unpaid post as President Donald Trump’s adviser on deregulation efforts, stepped down as The New Yorker was preparing to publish a lengthy article detailing Icahn’s potential conflicts of interest and questioning the legality of his actions. Icahn said in a letter to Trump released Friday that he would resign to prevent “partisan bickering” about his role that Democrats suggested could benefit him financially. The resignation came just three days before The New Yorker was scheduled to post its story online and begin selling printed magazines on newsstands.