ENH: Simplify DICOM plugin removing unneeded widget class.

This is made possible by previous commit (ENH: Extend module API
with "widgetRepresentationCreationEnabled" property)

Note that since the qSlicerScripedModule class can detect with a widget
class is available or not, there is no need to explicitly set
the "widgetRepresentationCreationEnabled" property.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24153 3bd1e089-480b-0410-8dfb-8563597acbee