Remove the cells_unique_claims feature flag

What does this MR do and why?

Removes cells_unique_claims, the global switch for the Cells claiming system, and its YAML definition. Claiming now depends only on the cell application setting, Gitlab.config.cell.enabled. Claiming has been live in production long enough that the global kill switch no longer earns its maintenance cost, and it's confirmed enabled on production.

The per model claiming FF has been removed in !251635 (merged).

References

• gitlab-com/gl-infra/tenant-scale/cells-infrastructure/team#737 (closed)

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 and then delete a group, confirming with gdk psql -d topology_service -c "SELECT * FROM claims;" that the route claim is created and then released, with no feature flag enabled beyond the cell configuration.

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