Tiny remove outdated comments (#130387)

I get confused when reading this comment. It seems that this code never calls `AnimationController.fling` (but only things like `animateTo`). Therefore, I do not think discussing the duration of `AnimationController.fling` is helpful - maybe it is just an outdated comment?

packages/flutter/lib/src/cupertino/route.dart

@@ -724,8 +724,6 @@ class _CupertinoBackGestureController<T> {
   /// [fractionalVelocity] as a fraction of screen width per second.
   void dragEnd(double velocity) {
     // Fling in the appropriate direction.
-    // AnimationController.fling is guaranteed to
-    // take at least one frame.
     //
     // This curve has been determined through rigorously eyeballing native iOS
     // animations.