Simplify SplitPairedAndUnpairedTool list:paired_or_unpaired dispatch

Now that the dataset collection manager rejects list:paired_or_unpaired
collections containing raw HDAs, every element_object reaching the
dispatch loop is guaranteed to be a paired_or_unpaired sub-collection.
Drop the defensive getattr fallback added in 07832df.

Author: mvdbeek

lib/galaxy/tools/__init__.py

@@ -4065,8 +4065,8 @@ def _handle_paired(dce):
                 _handle_paired(element)
         elif collection_type == "list:paired_or_unpaired":
             for element in collection.elements:
-                sub_elements = getattr(element.element_object, "elements", None)
-                if not sub_elements or len(sub_elements) == 1:
+                sub_collection = element.element_object
+                if len(sub_collection.elements) == 1:
                     _handle_unpaired(element)
                 else:
                     _handle_paired(element)