dont enumerate runtimes in project.json file

src/csharp/Grpc.Core.Tests/project.json

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

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

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

src/csharp/Grpc.Examples/project.json

src/csharp/Grpc.HealthCheck.Tests/project.json

src/csharp/Grpc.IntegrationTesting.Client/project.json

src/csharp/Grpc.IntegrationTesting.QpsWorker/project.json

src/csharp/Grpc.IntegrationTesting.Server/project.json

src/csharp/Grpc.IntegrationTesting.StressClient/project.json

src/csharp/Grpc.IntegrationTesting/project.json

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

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

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

...ates/src/csharp/Grpc.Examples.Tests/project.json.template

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

...s/src/csharp/Grpc.HealthCheck.Tests/project.json.template

...harp/Grpc.IntegrationTesting.Client/project.json.template

...p/Grpc.IntegrationTesting.QpsWorker/project.json.template

...harp/Grpc.IntegrationTesting.Server/project.json.template