Cut no-longer-accurate microtask reference in finalizeTree doc (#149941)

These listeners were removed in b564b4c (flutter#9143).

Author: gnprice

packages/flutter/lib/src/widgets/framework.dart

@@ -3255,9 +3255,6 @@ class BuildOwner {
   ///
   /// In debug mode, this also runs some sanity checks, for example checking for
   /// duplicate global keys.
-  ///
-  /// After the current call stack unwinds, a microtask that notifies listeners
-  /// about changes to global keys will run.
   @pragma('vm:notify-debugger-on-exception')
   void finalizeTree() {
     if (!kReleaseMode) {