cleanup(GCS+gRPC): remove unused functions

[coryan]

Remove functions mapping between REST and gRPC requess, responses, and
metadata. These were only used in tests after the last cleanup.

Part of the work for #6982

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #7038 (8ef4f75) into main (46c9e78) will increase coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             main    #7038     +/-   ##
=========================================
  Coverage   94.47%   94.47%             
=========================================
  Files        1306     1304      -2     
  Lines      113551   112300   -1251     
=========================================
- Hits       107281   106100   -1181     
+ Misses       6270     6200     -70     

Impacted Files	Coverage Δ
google/cloud/storage/internal/grpc_client.cc	92.39% <ø> (+2.35%)	⬆️
google/cloud/storage/internal/grpc_client.h	100.00% <ø> (ø)
...torage/internal/grpc_client_object_request_test.cc	100.00% <ø> (ø)
google/cloud/storage/internal/grpc_client_test.cc	100.00% <ø> (ø)
google/cloud/bigtable/async_read_stream_test.cc	97.32% <0.00%> (-0.67%)	⬇️
...le/cloud/internal/default_completion_queue_impl.cc	97.00% <0.00%> (-0.60%)	⬇️
google/cloud/storage/notification_metadata.h	98.14% <0.00%> (-0.07%)	⬇️
google/cloud/storage/internal/bucket_requests.h	100.00% <0.00%> (ø)
...gle/cloud/storage/internal/notification_requests.h	100.00% <0.00%> (ø)
... and 3 more

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