BUG: Fix crash when saving data

Commit r22095[1] intended to disable "unit" functionality by
preventing the associated logic from being loaded.

This commit comments the "unit" code associated with the selection node
to fix crash happening when saving data.

See http://www.na-mic.org/Bug/view.php?id=3176
Fixes #3176

[1] http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22095

Co-authored-by: Marine Clogenson <marine.clogenson@epfl.ch>

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22108 3bd1e089-480b-0410-8dfb-8563597acbee