Deprecate this repo's option validator code gen in favor of the runtime'src

- This removes traces of the option code generator in this repo, now
that the code has moved to dotnet/runtime. Unfortunately, the generator
in dotnet/runtime has currently dropped the use of the 'file' visibility
accessor in generated code, which leads to some conflicting type
definitions in this code base due to the use of IntervalsVisibleTo.
This surfaces as build warnings which are safe to ignore, since the
C# compiler ends up binding to the right thing. Fixing these warnings
will require a new drop of the runtime's generator.

Author: Martin Taillefer

eng/MSBuild/Generators.props

@@ -7,10 +7,6 @@
     <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.EnumStrings\Roslyn4.0\Microsoft.Gen.EnumStrings.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
   </ItemGroup>
 
-  <ItemGroup Condition="'$(UseOptionsValidationGenerator)' == 'true' and '$(BuildingSlnForDocs)' != 'true'">
-    <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.OptionsValidation\Roslyn4.0\Microsoft.Gen.OptionsValidation.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
-  </ItemGroup>
-
   <ItemGroup Condition="'$(UseMeteringGenerator)' == 'true' and '$(BuildingSlnForDocs)' != 'true'">
     <ProjectReference Include="$(MSBuildThisFileDirectory)\..\..\src\Generators\Microsoft.Gen.Metering\Roslyn4.0\Microsoft.Gen.Metering.Roslyn4.0.csproj" OutputItemType="Analyzer" ReferenceOutputAssembly="false" Private="false" IncludeAssets="runtime; build; native; contentfiles; analyzers; buildtransitive" />
   </ItemGroup>