fix unlearner does not erase rows correctly

Currently, NN-based recommender does not erase rows correctly.
Unlearner expects that the callback function (which is registered to the unlearner) does not change the unlearner state.

I changed the callback function from `clear_row` (which calls the `remove` method of unlearner) to `unlearn_row` (which is a new one, does not call `remove` method).