dump `kdtree.py`?

* Our stuff `kdtree.py` merely wraps `scipy.spatial.{c}KDTree` 
* `graph` uses `scipy.spatial.KDTree` [directly](https://github.com/pysal/libpysal/blob/011d7e617c55e987c3609633c770adea0ea3e507/libpysal/graph/_kernel.py#L280-L286)
* xref
  * #332 
  * #484  
  * https://github.com/pysal/pysal/issues/1428