[BE][AOTI] Remove duplicate schema for ExternKernelNode (#155867)

Summary: The definition of `ExternKernelNode` and `ExternKernelNodes` schema in `torch/_export/serde/aoti_schema.py` is a complete duplicate of the ones in `torch/_export/serde/schema.py`.

Test Plan:
CI

Rollback Plan:

Differential Revision: D76558294

Pull Request resolved: https://github.com/pytorch/pytorch/pull/155867
Approved by: https://github.com/jingsh

.github/labeler.yml

@@ -116,7 +116,6 @@
 "release notes: inductor (aoti)":
 - torch/_C/_aoti.pyi
 - torch/_dynamo/repro/aoti.py
-- torch/_export/serde/aoti_schema.py
 - torch/_higher_order_ops/aoti_call_delegate.py
 - torch/_inductor/codegen/aoti_runtime/**
 - torch/_inductor/codegen/aoti_hipify_utils.py

torch/_export/serde/aoti_schema.py

@@ -1,14 +0,0 @@
-from dataclasses import dataclass
-
-from torch._export.serde.schema import Node
-
-
-@dataclass
-class ExternKernelNode:
-    name: str
-    node: Node
-
-
-@dataclass
-class ExternKernelNodes:
-    nodes: list[ExternKernelNode]

torch/_inductor/extern_node_serializer.py

@@ -1,6 +1,6 @@
 import json
 
-from torch._export.serde.aoti_schema import ExternKernelNode, ExternKernelNodes, Node
+from torch._export.serde.schema import ExternKernelNode, ExternKernelNodes, Node
 from torch._export.serde.serialize import _dataclass_to_dict, EnumEncoder
 from torch._inductor.ir import ExternKernelNode as inductor_ExternKernelNode