DatastoreにLow Level APIでデータの削除をする

  private static void deleteModel(Entity model) throws DatastoreException {

      BeginTransactionRequest.Builder treq =
        BeginTransactionRequest.newBuilder();
      BeginTransactionResponse tres = datastore.beginTransaction(treq.build());
      ByteString tx = tres.getTransaction();

      CommitRequest.Builder request = CommitRequest.newBuilder();
      request.setTransaction(tx);
      Entity.Builder entity = Entity.newBuilder(lock);

      request.getMutationBuilder().addDelete(entity.getKey());

      CommitResponse response = datastore.commit(request.build());

      request.clearTransaction();
  }