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

[zhumin8]

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

[gemini-code-assist]

Code Review

This pull request cleans up the repository by removing a large number of files, scripts, templates, and tests related to the hermetic library generation tool, as well as updating configuration files and documentation to remove references to these deleted assets. Feedback on the changes highlights a PEP 8 style issue in compare_poms.py, where a function definition is incorrectly placed in the middle of the import block and a duplicate import of the sys module is introduced.

[gemini-code-assist]

To adhere to PEP 8, all imports should be placed at the top of the file, before any function definitions. Currently, eprint is defined before xml.etree.ElementTree and Counter are imported, and sys is imported twice (at the top and at the bottom). Please move the definition of eprint to the end of the import block (after the existing imports) and remove the duplicate import sys statement.

References

1. Imports are always put at the top of the file, just after any module comments and docstrings, and before module globals and constants. ^(link)