Use the default supported nuget versions. (#27622)

src/System.Diagnostics.DiagnosticSource/pkg/System.Diagnostics.DiagnosticSource.pkgproj

@@ -1,10 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
-  <PropertyGroup>
-    <!-- we need to be supported on pre-nuget-3 platforms (Dev12, Dev11, etc) -->
-    <MinClientVersion>2.8.6</MinClientVersion>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\src\System.Diagnostics.DiagnosticSource.csproj">
       <SupportedFramework>net46;net45;netcore45;netcoreapp1.0;wpa81;$(AllXamarinFrameworks)</SupportedFramework>

src/System.Reflection.Metadata/pkg/System.Reflection.Metadata.pkgproj

@@ -1,10 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
-  <PropertyGroup>
-    <!-- we need to be supported on pre-nuget-3 platforms (Dev12, Dev11, etc) -->
-    <MinClientVersion>2.8.6</MinClientVersion>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\src\System.Reflection.Metadata.csproj">
       <SupportedFramework>net45;netcore45;netcoreapp1.0;wpa81;$(AllXamarinFrameworks)</SupportedFramework>

src/System.Threading.Tasks.Dataflow/pkg/System.Threading.Tasks.Dataflow.pkgproj

@@ -1,10 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), dir.props))\dir.props" />
-  <PropertyGroup>
-    <!-- we need to be supported on pre-nuget-3 platforms (Dev12, Dev11, etc) -->
-    <MinClientVersion>2.8.6</MinClientVersion>
-  </PropertyGroup>
   <ItemGroup>
     <ProjectReference Include="..\src\System.Threading.Tasks.Dataflow.csproj">
       <SupportedFramework>net45;netcore45;wp8;wpa81;netcoreapp1.0;$(AllXamarinFrameworks)</SupportedFramework>