ENH: Remove "Load & Add Scenes Or Individual Datasets" section from Data module.

Minimize confusion by having on way of loading data.
See http://www.slicer.org/slicerWiki/index.php/Documentation/Nightly/SlicerApplication/LoadingData

See http://www.na-mic.org/Bug/view.php?id=2892

Fixes #2892

git-svn-id: http://svn.slicer.org/Slicer4/trunk@21646 3bd1e089-480b-0410-8dfb-8563597acbee