Remove Cells claiming feature flags

What does this MR do and why?

Removes 23 Cells claiming feature flags: 10 per-attribute claiming flags such as cells_claims_users, and 13 cells_claims_verification_worker_* flags, one per claimable model, such as cells_claims_verification_worker_user. All 23 are confirmed enabled on production via the public feature flag log at https://gitlab.com/gitlab-com/gl-infra/feature-flag-log, some since March 2026, so they no longer earn their keep.

One flag stays: cells_claims_service_desk_settings_project_key_address_slugs, along with the feature_flag: argument on ServiceDeskSetting#project_key_address_slug. It has never been enabled anywhere, not even in development. Its rollout, owned by group::work items, is still open at #606593; removing it now would enable the attribute in production with no rollout.

The feature_flag: argument is dropped from 13 of the 14 cells_claims_attribute declarations in app/models/. Cells::Claimable#cells_claims_enabled_for_attribute? already returns true for an unflagged attribute, so the concern is unchanged; the keyword stays on the DSL for future flags. Cells::ClaimsVerificationWorker#enabled? no longer derives a flag name from the model: it's now a fast-path pre-check that skips the exclusive lease when there's nothing to reconcile, with Cells::Claims::VerificationService remaining the authority per attribute.

This MR depends on !251634 (merged) merging first and targets that branch so its diff shows only its own changes; GitLab retargets it to master once the prerequisite merges.

References

• gitlab-com/gl-infra/tenant-scale/cells-infrastructure/team#737 (closed)
• !240942 (merged), a prior single-attribute cleanup

How to set up and validate locally

1. Set up cells in the GDK, per https://gitlab.com/gitlab-org/gitlab-development-kit/-/blob/main/doc/howto/cells.md#setting-up-cells-locally.
2. Create a user, a group and a project, then confirm claim rows appear via gdk psql -d topology_service -c "SELECT * FROM claims;".
3. Rename the group and confirm the old claim is released and a new one created.

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