get rid of the obsolete C# package builder

src/csharp/Grpc.Auth/Grpc.Auth.csproj

src/csharp/Grpc.Auth/Grpc.Auth.nuspec

src/csharp/Grpc.Core/Grpc.Core.csproj

src/csharp/Grpc.Core/Grpc.Core.nuspec

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.csproj

src/csharp/Grpc.HealthCheck/Grpc.HealthCheck.nuspec

src/csharp/Grpc.Reflection/Grpc.Reflection.csproj

src/csharp/Grpc.Reflection/Grpc.Reflection.nuspec

src/csharp/build_packages.bat

templates/src/csharp/build_packages.bat.template

tools/run_tests/package_targets.py