Removed obsolete docstring from WhereNode.clone().

Node.subtree_parents was removed in
d3f00bd570. That commit also added
Q.clone() which was identical to WhereNode.clone(), but lacked the
docstring. Q.clone() was later removed in
b454e2cbc9.

django/db/models/sql/where.py

@@ -171,11 +171,6 @@ class WhereNode(tree.Node):
                 self.children[pos] = child.relabeled_clone(change_map)
 
     def clone(self):
-        """
-        Create a clone of the tree. Must only be called on root nodes (nodes
-        with empty subtree_parents). Childs must be either (Constraint, lookup,
-        value) tuples, or objects supporting .clone().
-        """
         clone = self.__class__._new_instance(
             children=None,
             connector=self.connector,