Interop server should not implement "unimplementedCall"

[jtattermusch]

From the server.dart code it looks like that the dart interop server implements the "unimplementedCall" https://github.com/grpc/grpc-dart/blob/ae17e712e4f5431eb6664f0589899c77462e5475/interop/bin/server.dart#L117

This is actually not what's intended by the interop test spec. The idea is that if the server receives a request for a gRPC method it doesn't know about (no handler is registered for given method), it should return UNIMPLEMENTED (as kind of a "fallback" handler). The idea is not to implement a method that returns such status. See https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#unimplemented_method

also see https://github.com/grpc/grpc/blob/master/doc/interop-test-descriptions.md#unimplemented_service