Remove EXPERIMENTAL notice for DLQ b/c it's GA now (#87)

Author: pradn

google/cloud/pubsub_v1/subscriber/message.py

@@ -187,10 +187,6 @@ def delivery_attempt(self):
         The first delivery of a given message will have this value as 1. The value
         is calculated at best effort and is approximate.
 
-        EXPERIMENTAL: This feature is part of a closed alpha release. This
-        API might be changed in backward-incompatible ways and is not recommended
-        for production use. It is not subject to any SLA or deprecation policy.
-
         Returns:
             Optional[int]: The delivery attempt counter or None.
         """