remove outdated comment

Ix.NET/Source/Directory.build.targets

@@ -14,11 +14,6 @@
     <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1'">
-    <!--
-    NB: USE_ASYNC_ITERATOR relies on members in the framework only present in .NET Core 3.0 Preview 2 at the moment:
-          error CS0518: Predefined type 'System.Runtime.CompilerServices.AsyncIteratorMethodBuilder' is not defined or imported
-          error CS0656: Missing compiler required member 'System.Threading.Tasks.Sources.ManualResetValueTaskSourceCore`1.*
-    -->
     <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR;HAS_ASYNCENUMERABLE;HAS_ASYNCDISPOSABLE;BCL_HAS_CONFIGUREAWAIT</DefineConstants>
   </PropertyGroup>