Remove redundant publishers feature flag

[Sanket17052006]

Part of #12946

This PR removes the publishers feature flag since it's enabled across all environments.

Technical

• Removed publishers: enabled from conf/openlibrary.yml.
• Removed publishers field from pydantic-settings Features model in openlibrary/core/features.py.
• Removed is_enabled() overrides from publishers and index classes in openlibrary/plugins/worksearch/publishers.py — parent delegate.page defaults to True, matching the always-on behavior.
• Simplified openlibrary/templates/publishers/view.html and openlibrary/templates/type/edition/view.html by removing the "publishers" in ctx.features branches. The /publishers/<name> URL path is now the default — the old /search?publisher_facet=... fallback is deleted.
• Removed obsolete i18n strings from messages.pot and cleaned up test fixtures in test_features.py.

Testing

• make test-py-uv PYTEST_ARGS="openlibrary/tests/core/test_features.py"
• Grepped for "publishers" in ctx.features and "publishers" in web.ctx.features — zero remaining

Screenshot

N/A

Stakeholders

@RayBB

[openlibrary-bot]

Thanks for the PR!

The linked issue hasn't been triaged yet — triage happens on Mondays and Fridays. There are currently 24 open non-draft PRs ahead of yours.

PR triage checklist (maintainers / Richy)

• PR description — not empty; explains what the change does and how to verify it
• References an issue — PR body contains a #NNN reference
  • Linked issue is triaged — has a Priority: * label (not just Needs: Triage)
  • Linked issue is assigned — has at least one assignee
• Commit history clean — no WIP/fixup/conflict noise; commit messages are meaningful
• CI passing — no failing check-runs
• Test cases present — if the change touches substantive logic, test coverage exists or is explained
• Proof of testing — PR body includes a description of what was tested, a screenshot, or a video

Note

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[RayBB]

Very fast and good work!

Put it on testing and it's going excellent.

I also did a search and feel very sure there are no other usages.

It's so nice we are cleaning up our feature flags!