ENH: Remove redundant markups displayable manager method

This commit remove the method "GetSliceNode" and instead make
use of the existing method "GetMRMLSliceNode"

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23842 3bd1e089-480b-0410-8dfb-8563597acbee
COMP: Fix wrapping of Slicer/ThreeD displayable manager factory with VTKv6

This commit fixes a regression introduced by r23838 (which was known
to work with VTKv5)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@23843 3bd1e089-480b-0410-8dfb-8563597acbee