impl: remove deprecated retry_traits.h files

[mpeddada1]

For #9841

[gemini-code-assist]

Summary of Changes

Hello @mpeddada1, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request cleans up the codebase by removing several deprecated retry_traits.h header files across various Google Cloud C++ client libraries, specifically BigQuery, IAM, and Spanner admin. These headers were identified as exposing only internal symbols and are no longer necessary. The changes also include updates to the v3 migration guide to inform users of these removals and a corresponding adjustment to the Bazel build configuration for the Spanner library.

Highlights

• Deprecated Header Removal: Removed retry_traits.h files from the BigQuery, IAM, and Spanner admin modules, as these headers were deprecated and only exposed internal symbols.
• Documentation Update: Updated the doc/v3-migration-guide.md to include specific notes about the removal of these retry_traits.h files, clarifying that they contained internal symbols.
• Build Configuration Adjustment: Modified google/cloud/spanner/google_cloud_cpp_spanner.bzl to remove the reference to the now-deleted admin/retry_traits.h header.

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[gemini-code-assist]

Code Review

This pull request removes several deprecated retry_traits.h files and updates the build files and migration guide accordingly. The changes are correct and follow the intent of the associated issue. I've provided a few suggestions to improve the clarity and consistency of the documentation in doc/v3-migration-guide.md to make it easier for users to understand which files were removed for each service.

[gemini-code-assist]

For clarity, could you please specify that this change is for BigQuery? Since there isn't a dedicated ### BigQuery section, it's a bit ambiguous under ### Common. I'd suggest making the summary and body more specific.

Suggested change

	<summary>Removed <code>retry_traits.h</code> file</summary>
	The library no longer exposes the `retry_traits.h` header file. It only
	contained internal symbols.
	<summary>Removed <code>bigquery/retry_traits.h</code> file</summary>
	The library no longer exposes the `google/cloud/bigquery/retry_traits.h` header file. It only
	contained internal symbols.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (d0db317) to head (3d70999).

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15851      +/-   ##
======================================================
- Coverage               92.95%   92.94%   -0.01%     
======================================================
  Files                    2454     2454              
  Lines                  227401   227401              
======================================================
- Hits                   211380   211367      -13     
- Misses                  16021    16034      +13     

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