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

[eddiebergman]

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.

class Backend:

    def __del__(self):
        if delete_tmp_dir:
             os.rmdir(...)