The toString and toBSON functions of CollectionBulkLoader are not used by any other codes. Its subclass overrides these two functions in which serialize nss to string. It's impossible to get to know how to correctly serialize the nss in multitenancy environment as we enforce the caller to pass in serialization context or call `toStringFor*()` to indicate the target. 

We'd better to remove these two functions from the code base. It will be help us to keep the code clean and easy to be maintained.