Introduce flag to use pickle for serialization (#5805)

python/ray/cloudpickle/__init__.py

@@ -1,12 +1,7 @@
 from __future__ import absolute_import
 import sys
 
-# TODO(suquark): This is a temporary flag for
-# the new serialization implementation.
-# Remove it when the old one is deprecated.
-USE_NEW_SERIALIZER = False
-
-if USE_NEW_SERIALIZER and sys.version_info[:2] >= (3, 8):
+if sys.version_info[:2] >= (3, 8):
     from ray.cloudpickle.cloudpickle_fast import *
     FAST_CLOUDPICKLE_USED = True
 else: