Remove validation check for service fabric msbuild library (#330)

On VS 2019 latest version (16.11), msbuild on sfproj fails as it does not like the validation step. Removing that as the validation is only a check and not a necessity. Sample error: https://peservicepf2.visualstudio.com/servicefabric0224pf/_build/results?buildId=2519&view=results

dotnet/aspnetcore/servicefabric/Application/Voting/Voting.sfproj

@@ -1,5 +1,5 @@
 <?xml version="1.0" encoding="utf-8"?>
-<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003" InitialTargets=";ValidateMSBuildFiles">
+<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <Import Project="..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.props" Condition="Exists('..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.props')" />
   <PropertyGroup Label="Globals">
     <ProjectGuid>af958126-cb44-487a-8292-2d27594966de</ProjectGuid>
@@ -40,8 +40,4 @@
   </PropertyGroup>
   <Import Project="$(ApplicationProjectTargetsPath)" Condition="Exists('$(ApplicationProjectTargetsPath)')" />
   <Import Project="..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.targets" Condition="Exists('..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.targets')" />
-  <Target Name="ValidateMSBuildFiles">
-    <Error Condition="!Exists('..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.props')" Text="Unable to find the '..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.props' file. Please restore the 'Microsoft.VisualStudio.Azure.Fabric.MSBuild' Nuget package." />
-    <Error Condition="!Exists('..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.targets')" Text="Unable to find the '..\packages\Microsoft.VisualStudio.Azure.Fabric.MSBuild.1.6.4\build\Microsoft.VisualStudio.Azure.Fabric.Application.targets' file. Please restore the 'Microsoft.VisualStudio.Azure.Fabric.MSBuild' Nuget package." />
-  </Target>
-</Project>
+</Project>