cleanup of project.json files

src/csharp/Grpc.Auth/project.json

src/csharp/Grpc.Core/project.json

src/csharp/Grpc.Examples.MathClient/project.json

src/csharp/Grpc.Examples.MathServer/project.json

src/csharp/Grpc.Examples/project.json

src/csharp/Grpc.HealthCheck/project.json

templates/src/csharp/Grpc.Auth/project.json.template

templates/src/csharp/Grpc.Core/project.json.template

...src/csharp/Grpc.Examples.MathClient/project.json.template

...src/csharp/Grpc.Examples.MathServer/project.json.template

templates/src/csharp/Grpc.Examples/project.json.template

templates/src/csharp/Grpc.HealthCheck/project.json.template