Remove unused validation code from CollectionImpl::insertDocumentForBulkLoader which is only used by CollectionBulkLoader(initial sync collection cloner).

During initial sync collection cloning, a collection is created which disables document validation. So, even if there is an active validator, collection validation will be blocked always during initial sync. Therefore calling checkValidation during initial sync will always return Status::OK(). This code should be removed and replaced with an invariant that there is no validator or that document validation is disabled.