Fix `__del__` of backend so that it works as expected.

Previously, we used to delete the backend with `__del__` in `AutoML` but this was changed with PR #1155 due to issues of modules being unloaded before `__del__` had finished.

The real solution was to move `__del__` to the backend object instead of the `AutoML` object.

```python
class Backend:

    def __del__(self):
        if delete_tmp_dir:
             os.rmdir(...)
```