[Serve] Use ray's cloudpickle (#6051)

* Revert "Add cloudpickle as doc requirements (#6037)"

This reverts commit 03ce3b7c5b.

* Use ray's vendored cloudpickle

doc/requirements-doc.txt

@@ -23,4 +23,3 @@ flask
 uvicorn
 pygments
 werkzeug
-cloudpickle

python/ray/experimental/serve/kv_store_service.py

@@ -2,7 +2,7 @@ import json
 import sqlite3
 from abc import ABC
 
-import cloudpickle as pickle
+from ray import cloudpickle as pickle
 
 import ray.experimental.internal_kv as ray_kv
 from ray.experimental.serve.utils import logger