[cfe] Remove outdated TODO

Change-Id: If257f679942b8d9bf694b08684be338330858572
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/410420
Auto-Submit: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/front_end/lib/src/kernel/body_builder.dart

@@ -7149,8 +7149,6 @@ class BodyBuilder extends StackListenerImpl
   @override
   void handleNullAwareElement(Token nullAwareElement) {
     debugEvent("NullAwareElement");
-    // TODO(cstefantsova): Replace the following no-op with the node for
-    // handling null-aware elements.
     if (!libraryFeatures.nullAwareElements.isEnabled) {
       addProblem(
           templateExperimentNotEnabledOffByDefault