nit: remove redundant/wrong Pub/Sub region tag [(#4027)](GoogleCloudPlatform/python-docs-samples#4027)

Author: anguillanneuf

samples/snippets/subscriber.py

@@ -348,7 +348,6 @@ def callback(message):
 
 def receive_messages_with_custom_attributes(project_id, subscription_id, timeout=None):
     """Receives messages from a pull subscription."""
-    # [START pubsub_subscriber_sync_pull_custom_attributes]
     # [START pubsub_subscriber_async_pull_custom_attributes]
     from concurrent.futures import TimeoutError
     from google.cloud import pubsub_v1
@@ -383,7 +382,6 @@ def callback(message):
         except TimeoutError:
             streaming_pull_future.cancel()
     # [END pubsub_subscriber_async_pull_custom_attributes]
-    # [END pubsub_subscriber_sync_pull_custom_attributes]
 
 
 def receive_messages_with_flow_control(project_id, subscription_id, timeout=None):