chore: migrate google-storagetransfer to librarian

[jskeet]

This PR migrates the google-storagetransfer package to use Librarian for code generation.

Highlights

• Enabled Librarian generation for google-storagetransfer by removing skip_generate: true in librarian.yaml.
• Added keep configuration for sample test utilities (samples/test/utils/bucket.js, samples/test/utils/index.js, samples/test/utils/sample.js, samples/test/utils/transfer.js) to preserve them during generation.
• Removed OwlBot configuration (.OwlBot.yaml).
• Updated .repo-metadata.json (pretty name, description, documentation links, and removed requires_billing).

Note on samples-test Task

The preserved utility files in samples/test/utils/ are intended for use by the sample test samples/test/quickstart.js.

These utility files are preserved for documentation purposes for library users, ensuring the sample tests remain complete and instructive.

Note that the samples-test task (npm run samples-test) currently fails to execute this test due to a wildcard mismatch in samples/package.json (which looks for test/*.test.js while the test file is quickstart.js).

User-facing Changes

None.

[gemini-code-assist]

Code Review

This pull request enables code generation for the Google Storage Transfer service by removing the skip_generate flag in librarian.yaml. It also updates the repository metadata in .repo-metadata.json and deletes several unused sample helper and configuration files. There are no review comments, so no further feedback is provided.

[jskeet]

@quirogas I've added the samples/test/utils files to the keep list.