Remove obsolete extensions VisuaLine and PathPlanner

As discussed with @lchauvin, PathXplorer extension will be added shortly.
It will regroup VisuaLine and PathPlanner. Therefore, there is no need
to maintain these extensions anymore.

See https://github.com/SlicerIGT/SlicerIGT/issues/13#issuecomment-29928719
Merge pull request #504 from jcfr/remove-obsolete-extensions

Remove obsolete extensions VisuaLine and PathPlanner