SPRINGFIELD — The state’s Commission on Government Forecasting and Accountability has decreased its current-year budget forecast by $728 million, erasing much of a once-predicted surplus that had led Gov. J.B. Pritzker to float the idea of tax cuts earlier this spring.While the commission’s $51.2 million estimate is now 1.2 percent below its estimate from early March, the impact on Pritzker’s budget proposal is relatively small because his plan relied on a more conservative estimate.&ldquo …