Remove unused #defines for Ix.

Ix.NET/Source/Directory.build.targets

@@ -22,10 +22,10 @@
     <DefineConstants>$(DefineConstants);NO_ARRAY_EMPTY;NO_TASK_FROMEXCEPTION</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'net46' or '$(TargetFramework)' == 'net461'">
-    <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR</DefineConstants>
+    <DefineConstants>$(DefineConstants)</DefineConstants>
   </PropertyGroup>
   <PropertyGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0' or '$(TargetFramework)' == 'netstandard2.1' or '$(TargetFramework)' == 'netstandard2.0'">
-    <DefineConstants>$(DefineConstants);USE_ASYNC_ITERATOR;HAS_ASYNCENUMERABLE;HAS_ASYNCDISPOSABLE;BCL_HAS_CONFIGUREAWAIT;HAS_VALUETUPLE</DefineConstants>
+    <DefineConstants>$(DefineConstants);HAS_VALUETUPLE</DefineConstants>
   </PropertyGroup>
 
   <PropertyGroup>

Ix.NET/Source/System.Linq.Async/System/Diagnostics/StackTraceHiddenAttribute.cs

@@ -1,16 +0,0 @@
-#if !BCL_HAS_CONFIGUREAWAIT
-
-using System;
-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-// See the LICENSE file in the project root for more information.
-
-namespace System.Diagnostics
-{
-    [AttributeUsage(AttributeTargets.Class | AttributeTargets.Method | AttributeTargets.Constructor | AttributeTargets.Struct, Inherited = false)]
-    internal sealed class StackTraceHiddenAttribute : Attribute
-    {
-        public StackTraceHiddenAttribute() { }
-    }
-}
-#endif