SPRINGFIELD — It spurred litigation between Chicago schools and Gov. Bruce Rauner’s administration. It begat a bizarre, politically tinged summit between Rauner and Grammy-winning hip-hop artist Chance the Rapper and it remains a sticking point in negotiations to end the state’s nearly two-year-old budget impasse.A school funding deal former Chicago mayor Richard M. Daley and GOP lawmakers cut more than 20 years ago has caused turmoil in Illinois this year.The provision enshrined in the state pension code …