removed unneeded retrieval of head node

Remove the code that tries to get the `head` node, since it is not even needed.  Fixes #38

Although it is still true that the `head` node is missing at the root of the Feature Structure that get passed, and `head-word` is pointing to a null value (again, at the root).

The only places where `head` and `head-word` is correct is at the node with`<HEAD-FLAG> = T`.  This correct 'head', and 'head-word' do not propagate to the other node (I think since `F_R` do not points to the node with `<HEAD-FLAG> = T` for most other node).