impl(spanner): remove deprecated Admin classes (googleapis#15862)

* impl(spanner): remove deprecated Admin classes

Author: mpeddada1

ci/abi-dumps/google_cloud_cpp_spanner.expected.abi.dump.gz

@@ -347,6 +347,44 @@ file. It only contained internal symbols.
 
 </details>
 
+<details>
+<summary>Removed Admin Clients from <code>spanner</code> namespace</summary>
+
+The `DatabaseAdminClient` and `InstanceAdminClient` classes (and their
+associated connection classes and factory functions) have been removed from the
+`google::cloud::spanner` namespace. Please use the replacements in
+`google::cloud::spanner_admin`.
+
+**Before:**
+
+```cpp
+#include "google/cloud/spanner/database_admin_client.h"
+#include "google/cloud/spanner/instance_admin_client.h"
+
+namespace spanner = ::google::cloud::spanner;
+
+void Function(spanner::DatabaseAdminClient db_admin,
+              spanner::InstanceAdminClient in_admin) {
+  // ...
+}
+```
+
+**After:**
+
+```cpp
+#include "google/cloud/spanner/admin/database_admin_client.h"
+#include "google/cloud/spanner/admin/instance_admin_client.h"
+
+namespace spanner_admin = ::google::cloud::spanner_admin;
+
+void Function(spanner_admin::DatabaseAdminClient db_admin,
+              spanner_admin::InstanceAdminClient in_admin) {
+  // ...
+}
+```
+
+</details>
+
 ### Storage
 
 <details>

doc/v3-migration-guide.md

@@ -106,20 +106,12 @@ add_library(
     create_instance_request_builder.h
     database.cc
     database.h
-    database_admin_client.cc
-    database_admin_client.h
-    database_admin_connection.cc
-    database_admin_connection.h
     date.h
     directed_read_replicas.h
     encryption_config.h
     iam_updater.h
     instance.cc
     instance.h
-    instance_admin_client.cc
-    instance_admin_client.h
-    instance_admin_connection.cc
-    instance_admin_connection.h
     internal/channel.h
     internal/connection_impl.cc
     internal/connection_impl.h
@@ -361,8 +353,6 @@ if (GOOGLE_CLOUD_CPP_WITH_MOCKS)
         # cmake-format: sort
         admin/mocks/mock_database_admin_connection.h
         admin/mocks/mock_instance_admin_connection.h
-        mocks/mock_database_admin_connection.h
-        mocks/mock_instance_admin_connection.h
         mocks/mock_spanner_connection.h
         mocks/row.h)
     export_list_to_bazel("google_cloud_cpp_spanner_mocks.bzl"
@@ -458,11 +448,7 @@ function (spanner_client_define_tests)
         commit_options_test.cc
         connection_options_test.cc
         create_instance_request_builder_test.cc
-        database_admin_client_test.cc
-        database_admin_connection_test.cc
         database_test.cc
-        instance_admin_client_test.cc
-        instance_admin_connection_test.cc
         instance_test.cc
         internal/connection_impl_test.cc
         internal/database_admin_logging_test.cc