Remove Checkpoint.writes

- This has been superseded by saving the individual writes of each task through put_writes()
- Removing this speeds up checkpoint operations as it was duplicating data saved elsewhere already

libs/prebuilt/tests/test_react_agent.py

@@ -90,7 +90,6 @@ def test_no_prompt(sync_checkpointer: BaseCheckpointSaver, version: str) -> None
     assert saved.metadata == {
         "parents": {},
         "source": "loop",
-        "writes": {"agent": {"messages": [AIMessage(content="hi?", id="0")]}},
         "step": 1,
         "thread_id": "123",
     }
@@ -118,7 +117,6 @@ async def test_no_prompt_async(async_checkpointer: BaseCheckpointSaver) -> None:
     assert saved.metadata == {
         "parents": {},
         "source": "loop",
-        "writes": {"agent": {"messages": [AIMessage(content="hi?", id="0")]}},
         "step": 1,
         "thread_id": "123",
     }