remove the ImmutableCollections dependency

src/csharp/Grpc.Core/ChannelOptions.cs

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

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

src/csharp/Grpc.Core/KeyCertificatePair.cs

src/csharp/Grpc.Core/Metadata.cs

src/csharp/Grpc.Core/ServerCredentials.cs

src/csharp/Grpc.Core/ServerServiceDefinition.cs

src/csharp/Grpc.Core/packages.config

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

src/csharp/Grpc.IntegrationTesting/TestCredentials.cs

src/csharp/Grpc.IntegrationTesting/packages.config