SPRINGFIELD — The Illinois Supreme Court is deciding whether the court system can review a governor’s firing, particularly of a member of the Prisoner Review Board.Eric Gregg, formerly the mayor of Harrisburg, was appointed to the Prisoner Review Board by Gov. Patrick J. Quinn in 2013. Two years later, Rauner removed him from the position after the governor’s office said he was earning outside income — a violation of state statute.Under Illinois law, prisoner review board members — which …