BUG: Proceed to MarkupsFiducial picking manager configuration only for VTK6.

This commit fixes regression introduced in r23648 that was preventing
Slicer from being compiled against VTK5.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23665 3bd1e089-480b-0410-8dfb-8563597acbee