Make the multitargetting on netstandard2.0 working.

Need to understand more on why netstandard1.6 does not compile.

src/FSharp/FSharp-2017.fsproj

@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk" ToolsVersion="15.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
-    <TargetFrameworks>net45;netstandard1.6</TargetFrameworks>
-    <DisableImplicitFSharpCoreReference>true</DisableImplicitFSharpCoreReference>
+    <TargetFrameworks>net45;netstandard2.0</TargetFrameworks>
+    <GenerateAssemblyInfo>false</GenerateAssemblyInfo>
     <AssemblyName>MathNet.Numerics.FSharp</AssemblyName>
     <RootNamespace>MathNet.Numerics</RootNamespace>
     <GenerateDocumentationFile>true</GenerateDocumentationFile>
@@ -10,27 +10,10 @@
   </PropertyGroup>
 
   <PropertyGroup Condition="'$(TargetFramework)'=='netstandard1.6'">
-      <DefineConstants>NETSTANDARD;NOSYSNUMERICS</DefineConstants>
+    <DefineConstants>NETSTANDARD;NOSYSNUMERICS</DefineConstants>
   </PropertyGroup>
 
-  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
-      <DebugSymbols>true</DebugSymbols>
-      <DebugType>full</DebugType>
-      <Optimize>false</Optimize>
-      <Tailcalls>false</Tailcalls>
-      <OutputPath>bin\Debug\</OutputPath>
-  </PropertyGroup>
-  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
-      <DebugType>pdbonly</DebugType>
-      <Optimize>true</Optimize>
-      <Tailcalls>true</Tailcalls>
-      <OutputPath>bin\Release\</OutputPath>
-  </PropertyGroup>
-
-  <Import Project="$(FSharpTargetsPath)" Condition="Exists('$(FSharpTargetsPath)')" />
   <ItemGroup>
-    <PackageReference Include="FSharp.NET.Sdk" Version="1.0.*" PrivateAssets="All" />
-    <PackageReference Include="FSharp.Core" Version="4.2.*" PrivateAssets="All" />
     <ProjectReference Include="..\Numerics\Numerics.csproj" />
   </ItemGroup>
   <ItemGroup>