impl: remove deprecated retry_traits.h files

doc/v3-migration-guide.md

@@ -44,6 +44,14 @@ module which can be added to your `MODULE.bazel` file as a dependency.
 
 ### Common
 
+<details>
+<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.
+
+</details>
+
 ### Bigtable
 
 <details>
@@ -139,4 +147,22 @@ guide:
 
 ### Spanner
 
+<details>
+<summary>Removed <code>admin/retry_traits.h</code> file</summary>
+
+The library no longer exposes `google/cloud/spanner/admin/retry_traits.h` header
+file. It only contained internal symbols.
+
+</details>
+
 ### Storage
+
+### IAM
+
+<details>
+<summary>Removed <code>iam/retry_traits.h</code> file</summary>
+
+The library no longer exposes `google/cloud/iam/retry_traits.h` header file. It
+only contained internal symbols.
+
+</details>

google/cloud/spanner/CMakeLists.txt

@@ -88,7 +88,6 @@ add_library(
     admin/internal/instance_admin_tracing_connection.h
     admin/internal/instance_admin_tracing_stub.cc
     admin/internal/instance_admin_tracing_stub.h
-    admin/retry_traits.h
     backoff_policy.h
     backup.cc
     backup.h

google/cloud/spanner/google_cloud_cpp_spanner.bzl

@@ -45,7 +45,6 @@ google_cloud_cpp_spanner_hdrs = [
     "admin/internal/instance_admin_stub_factory.h",
     "admin/internal/instance_admin_tracing_connection.h",
     "admin/internal/instance_admin_tracing_stub.h",
-    "admin/retry_traits.h",
     "backoff_policy.h",
     "backup.h",
     "batch_dml_result.h",