[serve] Remove _prevent_downscale_ref in proxy (ray-project#42276)

No longer needed due to Ray autoscaler being changed to not remove nodes containing 0-CPU actors.

Signed-off-by: Edward Oakes <ed.nmi.oakes@gmail.com>

python/ray/serve/_private/proxy.py

@@ -226,9 +226,6 @@ def __init__(
             }
         )
 
-        # `self._prevent_node_downscale_ref` is used to prevent the node from being
-        # downscaled when there are ongoing requests
-        self._prevent_node_downscale_ref = ray.put("prevent_node_downscale_object")
         # `self._ongoing_requests` is used to count the number of ongoing requests
         self._ongoing_requests = 0
         # The time when the node starts to drain.