Chapter06/binary_search_tree - fix the case where node is not found.

[amirsoroush]

In Chapter06/binary_search_tree.py, the current implementation of get_node_with_parent has two problems:

1. The last line is unreachable.
2. In case the removing item is not present in the tree, an exception is raised. That's because current will eventually be None and None doesn't have .data attribute. So I returned the same (None, None) value so that the .remove() method terminates.