Revert "Avoid duplicate NullableAttributes.cs"

This reverts commit d5183ec.

Author: dahlbyk

src/libraries/Directory.Build.targets

@@ -170,7 +170,6 @@
   <!-- Adds Nullable annotation attributes to C# non .NETCoreApp builds. -->
   <ItemGroup Condition="'$(Nullable)' != '' and
                         '$(Nullable)' != 'disable' and
-                        '$(NoIncludeNullableAttributes)' != 'true' and
                         '$(MSBuildProjectExtension)' == '.csproj' and
                         '$(TargetFrameworkIdentifier)' != '.NETCoreApp'">
     <Compile Include="$(CoreLibSharedDir)System\Diagnostics\CodeAnalysis\NullableAttributes.cs" Link="System\Diagnostics\CodeAnalysis\NullableAttributes.cs" />

src/libraries/Microsoft.Extensions.Configuration.Binder/tests/SourceGenerationTests/Microsoft.Extensions.Configuration.Binder.SourceGeneration.Tests.csproj

@@ -8,9 +8,6 @@
     <NoWarn>$(NoWarn);SYSLIB1103,SYSLIB1104</NoWarn>
     <InterceptorsPreviewNamespaces>$(InterceptorsPreviewNamespaces);Microsoft.Extensions.Configuration.Binder.SourceGeneration</InterceptorsPreviewNamespaces>
     <EnableConfigurationBindingGenerator>true</EnableConfigurationBindingGenerator>
-
-    <!-- Avoid conflict with Microsoft.Extensions.Configuration.Binder (via InternalsVisibleTo) -->
-    <NoIncludeNullableAttributes>true</NoIncludeNullableAttributes>
   </PropertyGroup>
 
   <PropertyGroup>