upgrade Grpc.Auth to latest version of Google.Apis.Auth and remove unneeded code

src/csharp/Grpc.Auth/GoogleCredential.cs

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

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

src/csharp/Grpc.Auth/app.config

src/csharp/Grpc.Auth/packages.config

src/csharp/Grpc.IntegrationTesting.Client/app.config

src/csharp/Grpc.IntegrationTesting.Server/app.config

...rp/Grpc.IntegrationTesting/Grpc.IntegrationTesting.csproj

src/csharp/Grpc.IntegrationTesting/app.config

src/csharp/Grpc.IntegrationTesting/packages.config