revert(graph-engine): rollback stop-event unification (#32789)

api/core/workflow/runtime/graph_runtime_state.py

@@ -2,7 +2,6 @@ from __future__ import annotations
 
 import importlib
 import json
-import threading
 from collections.abc import Mapping, Sequence
 from copy import deepcopy
 from dataclasses import dataclass
@@ -219,8 +218,6 @@ class GraphRuntimeState:
         self._pending_graph_node_states: dict[str, NodeState] | None = None
         self._pending_graph_edge_states: dict[str, NodeState] | None = None
 
-        self.stop_event: threading.Event = threading.Event()
-
         if graph is not None:
             self.attach_graph(graph)