chore(hermetic_build): remove obsolete hermetic build source code and workflows (#13616)

Remove the old hermetic_build tool and workflows since the project has
migrated to Librarian for library generation. This change keeps
`google-cloud-java/sdk-platform-java/hermetic_build/library_generation/owlbot`
intact, will do a followup to cleanup bits inside `owlbot` package that
is not used anymore.

- Delete `common/` and `release_note_generation/` directories under
`sdk-platform-java/hermetic_build/`.
- Remove all files under
`sdk-platform-java/hermetic_build/library_generation/` except the
`owlbot/` package, `setup.py`, `requirements.txt`/`requirements.in`, and
`__init__.py`.
- Move the POM comparison helper script `compare_poms.py` into
`tests/owlbot/` to keep the remaining Owlbot test suite fully
self-contained.
- Delete unused build/test workflows, composite action, and scripts
(`hermetic-build-scripts-ci.yaml`,`.cloudbuild/library_generation/`,
`.github/scripts/action.yaml`, and
`.github/scripts/hermetic_library_generation.sh`).
- Update `release-please-config.json` and `AGENTS.md` references to
reflect the removed files.

For googleapis/librarian#5237

Author: zhumin8

.github/workflows/hermetic-build-scripts-ci.yaml

@@ -58,4 +58,3 @@ This is a standard Maven project.
 For development on core components, refer to the following guides in `sdk-platform-java`:
 - **GAPIC Generator**: [sdk-platform-java/gapic-generator-java/DEVELOPMENT.md](sdk-platform-java/gapic-generator-java/DEVELOPMENT.md)
 - **GAX**: [sdk-platform-java/gax-java/DEVELOPMENT.md](sdk-platform-java/gax-java/DEVELOPMENT.md)
-- **Hermetic Build**: [sdk-platform-java/hermetic_build/DEVELOPMENT.md](sdk-platform-java/hermetic_build/DEVELOPMENT.md)

AGENTS.md

@@ -10,14 +10,10 @@
       "extra-files": [
         "WORKSPACE",
         ".github/workflows/generated_files_sync.yaml",
-        ".github/workflows/hermetic_library_generation.yaml",
         "sdk-platform-java/.cloudbuild/graalvm/cloudbuild.yaml",
         "sdk-platform-java/.cloudbuild/graalvm/cloudbuild-test-a.yaml",
         "sdk-platform-java/.cloudbuild/graalvm/cloudbuild-test-b.yaml",
         "sdk-platform-java/.cloudbuild/graalvm/cloudbuild-test-c.yaml",
-        "sdk-platform-java/.cloudbuild/library_generation/cloudbuild-library-generation-push.yaml",
-        "sdk-platform-java/.cloudbuild/library_generation/library_generation.Dockerfile",
-        "sdk-platform-java/.cloudbuild/library_generation/library_generation_airlock.Dockerfile",
         "sdk-platform-java/hermetic_build/library_generation/owlbot/templates/java_library/.kokoro/presubmit/graalvm-native-a.cfg",
         "sdk-platform-java/hermetic_build/library_generation/owlbot/templates/java_library/.kokoro/presubmit/graalvm-native-b.cfg",
         "sdk-platform-java/hermetic_build/library_generation/owlbot/templates/java_library/.kokoro/presubmit/graalvm-native-c.cfg",