BF: remove unneeded VoxelPosition class from surfing.volsurf

The offending VoxelPosition class is not used, yet as it used the Python>=2.7 functools.total_ordering module - causing an error when using Python 2.6. 

This PR just removes the offending code.

Thanks to Payal Chakraborty for bringing up this issue.
http://lists.alioth.debian.org/pipermail/pkg-exppsy-pymvpa/2014q1/002682.html