refactor: remove dead Diagram.topo_sort() wrapper

Never called — __iter__ and __reversed__ use the module-level
topo_sort() directly. No public callers in the codebase.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/diagram.py

@@ -308,17 +308,6 @@ def __mul__(self, arg) -> "Diagram":
         self.nodes_to_show.intersection_update(arg.nodes_to_show)
         return self
 
-    def topo_sort(self) -> list[str]:
-        """
-        Return nodes in topological order.
-
-        Returns
-        -------
-        list[str]
-            Node names in topological order.
-        """
-        return topo_sort(self)
-
     @classmethod
     def cascade(cls, table_expr, part_integrity="enforce"):
         """