SPRINGFIELD — A lawsuit over unpaid plumbing fees has forced the state’s top court to throw part of its own precedent down the drain. The Illinois Supreme Court ruled Friday that a trial court’s judgment for a debt-collection agency was not void, despite the fact the agency was not licensed to collect under state law when it filed suit. In the process, it overturned part of its own decision from 10 years ago that said courts must have “inherent power” to decide certain matters. The 6 …