cleanup(common)!: remove legacy IAM classes

[dbolduc]

Fixes #5929

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Merging #8667 (e757965) into main (7bac6b9) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             main    #8667    +/-   ##
========================================
  Coverage   93.41%   93.42%            
========================================
  Files        1461     1456     -5     
  Lines      123977   123808   -169     
========================================
- Hits       115815   115666   -149     
+ Misses       8162     8142    -20     

Impacted Files	Coverage Δ
google/cloud/bigtable/instance_admin.h	100.00% <ø> (ø)
google/cloud/bigtable/table_admin.h	100.00% <ø> (ø)
google/cloud/bigtable/version.h	100.00% <ø> (ø)
google/cloud/storage/client.h	99.79% <ø> (ø)
...le/cloud/internal/default_completion_queue_impl.cc	97.15% <0.00%> (-0.57%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.00% <0.00%> (-0.25%)	⬇️
google/cloud/completion_queue_test.cc	97.13% <0.00%> (+0.19%)	⬆️
google/cloud/pubsub/subscriber_connection_test.cc	97.91% <0.00%> (+0.69%)	⬆️
...e/cloud/pubsublite/internal/alarm_registry_impl.cc	100.00% <0.00%> (+2.94%)	⬆️

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[google-cloud-cpp-bot]

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For commit: 419a3b15f2a05f7e64d4eaa323025394c195f970

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[google-cloud-cpp-bot]

Google Cloud Build Logs
For commit: e75796565f98bac98c15f29ca7ff9da1f34d6ba0

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