Drop a workaround for a resolved issue (dart-lang/stack_trace#63)

https://github.com/dart-lang/sdk/issues/14075 has been closed for a long
time.

This class in not in the public interface for the package so it should
not be breaking to remove the throwing setter.
diff --git a/pkgs/stack_trace/lib/src/lazy_trace.dart b/pkgs/stack_trace/lib/src/lazy_trace.dart
index 917d20e..0fa2af1 100644
--- a/pkgs/stack_trace/lib/src/lazy_trace.dart
+++ b/pkgs/stack_trace/lib/src/lazy_trace.dart
@@ -32,7 +32,4 @@
       LazyTrace(() => _trace.foldFrames(predicate, terse: terse));
   @override
   String toString() => _trace.toString();
-
-  // Work around issue 14075.
-  set frames(_) => throw UnimplementedError();
 }