fix(pubsub): no warnings on ack()/nack() success

[coryan]

Fixes #10921

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This change is 

[codecov]

Codecov Report

Base: 93.64% // Head: 93.65% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (7c598dd) compared to base (1f13831).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10920   +/-   ##
=======================================
  Coverage   93.64%   93.65%           
=======================================
  Files        1717     1717           
  Lines      154743   154762   +19     
=======================================
+ Hits       144914   144940   +26     
+ Misses       9829     9822    -7     

Impacted Files	Coverage Δ
...oogle/cloud/pubsub/internal/ack_handler_wrapper.cc	100.00% <100.00%> (ø)
.../cloud/pubsub/internal/ack_handler_wrapper_test.cc	100.00% <100.00%> (ø)
google/cloud/internal/timer_queue.cc	96.10% <0.00%> (-1.30%)	⬇️
google/cloud/storage/parallel_upload.cc	98.28% <0.00%> (-0.35%)	⬇️
google/cloud/pubsub/samples/samples.cc	87.01% <0.00%> (+0.07%)	⬆️
...le/cloud/storage/internal/curl_download_request.cc	89.29% <0.00%> (+0.33%)	⬆️
...cloud/pubsub/internal/subscription_session_test.cc	98.66% <0.00%> (+0.83%)	⬆️
...bigtable/examples/bigtable_hello_instance_admin.cc	83.00% <0.00%> (+2.00%)	⬆️

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