[cfe/vm] Remove outdated comment

This is a follow-up to
https://dart-review.googlesource.com/c/sdk/+/433480/comment/7c643917_f6b72121/

TEST=existing

Change-Id: Id5572f6f4d3a1ae71ce547e297e92193ffa058c3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/438800
Auto-Submit: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>

pkg/vm/lib/transformations/type_flow/summary_collector.dart

@@ -1597,7 +1597,6 @@ class SummaryCollector extends RecursiveResultVisitor<TypeExpr?> {
     }
   }
 
-  // Tests subtypes ignoring any nullabilities.
   bool _isSubtype(DartType subtype, DartType supertype) =>
       _environment.isSubtypeOf(subtype, supertype);