chore(hermetic_build): remove obsolete pom, clirr, and license headers logic from owlbot

[zhumin8]

Removes the related custom Python scripts, Jinja templates, and test fixtures from the hermetic build OwlBot codebase. This is a cleanup since clirr, pom.xml generation, and license headers are now handled by Librarian.

- Deleted:
  - `fix_poms.py`
  - `fix-license-headers.py`
  - `write_clirr_ignore.sh`
  - `compare_poms.py`
  - Unused templates under `templates/clirr` and `templates/poms`
  - Unused unit tests and test fixtures
- Modified:
  - `entrypoint.sh` to remove execution steps for deleted scripts
  - `setup.py` to clean up package_data listings

Fixes googleapis/librarian#5237

[gemini-code-assist]

Code Review

This pull request removes the OwlBot POM fixing, Clirr ignore generation, and license header fixing functionalities from the library generation pipeline. This includes deleting scripts like fix_poms.py, write_clirr_ignore.sh, and fix-license-headers.py, along with their associated Jinja templates, tests, and test resources. The entrypoint.sh and setup.py files have been updated to reflect these removals. There are no review comments provided, so I have no additional feedback.

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate passed for 'gapic-generator-java-root'

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[zhumin8]

Out of cautious, manually triggered generate workflow on this change: https://github.com/googleapis/google-cloud-java/actions/runs/28545135552/job/84628424599