Remove special flattening-behavior for Wizard content providers

This removes the feature from the "New" and "Import/Export" wizards,
where container for wizard elements would be flattened, if it only
contains a single, other container.

There are two reasons for this change:
- it is not consistent with the other viewers, that always show the
complete tree path (e.g. the preference dialog).
- it doesn't work properly, as it is still possible to produce the
undesirable behavior in the "New" wizard (for e.g. Java/JUnit/...)

See eclipse-platform#2602
for context.

bundles/org.eclipse.ui.workbench/eclipseui/org/eclipse/ui/internal/dialogs/WizardContentProvider.java

@@ -46,11 +46,6 @@ public Object[] getChildren(Object parentElement) {
 				handleChild(childWizard, list);
 			}
 
-			// flatten lists with only one category
-			if (list.size() == 1 && list.get(0) instanceof WizardCollectionElement) {
-				return getChildren(list.get(0));
-			}
-
 			return list.toArray();
 		} else if (parentElement instanceof AdaptableList) {
 			AdaptableList aList = (AdaptableList) parentElement;
@@ -59,10 +54,6 @@ public Object[] getChildren(Object parentElement) {
 			for (Object element : children) {
 				handleChild(element, list);
 			}
-			// if there is only one category, return it's children directly (flatten list)
-			if (list.size() == 1 && list.get(0) instanceof WizardCollectionElement) {
-				return getChildren(list.get(0));
-			}
 
 			return list.toArray();
 		} else {