Remove obsolete source stepping code

After merging the cross plot with the summary plot, the sources stepping can be simplified. Remove obsolete sources stepping code, to prepare for other source stepping issues.

ApplicationLibCode/ProjectDataModel/RimMainPlotCollection.cpp

@@ -210,7 +210,7 @@ void RimMainPlotCollection::initAfterRead()
         auto summaryPlot = new RimSummaryPlot;
         summaryMultiPlot->addPlot( summaryPlot );
 
-        for ( auto curve : crossPlot->allCurves( RimSummaryDataSourceStepping::Axis::Y_AXIS ) )
+        for ( auto curve : crossPlot->allCurves() )
         {
             crossPlot->removeCurve( curve );