Remove fix for bug in project system

src/csharp/Grpc.Tools/build/_protobuf/Google.Protobuf.Tools.targets

@@ -39,16 +39,6 @@
     <AvailableItemName Include="Protobuf" />
   </ItemGroup>
 
-  <!-- Design Time Build integration for Visual Studio -->
-  <ItemGroup Condition=" '$(DisableProtobufDesignTimeBuild)' != 'true' " >
-    <!-- Add Protobuf items to Content if they are not added yet -->
-    <UnaddedProtobufContent Include="@(Protobuf)" Exclude="@(Content)" />
-    <Content Include="@(UnaddedProtobufContent)" />
-
-    <!-- Remove Protobuf items from None -->
-    <None Remove="@(Protobuf)" />
-  </ItemGroup>
-
   <PropertyGroup>
     <!-- NET SDK: by default, do not include proto files in the directory.
          Current Microsoft's recommendation is against globbing: