Remove the Service Desk project key address slug claims flag

What does this MR do and why?

Removes the cells_claims_service_desk_settings_project_key_address_slugs feature flag, which gated Topology Service claims for ServiceDeskSetting#project_key_address_slug.

The flag is enabled on GitLab.com, and the claims verification worker has backfilled claims for the existing records. So the flag has done its job for the rollout. This follows the cleanup step in doc/development/cells/claims.md. The flag was left in place by !251635 (merged) only because it hadn't been enabled yet.

• Drops the feature_flag: argument from the project_key_address_slug claim declaration. Claiming for the attribute is now controlled only by the cell configuration, the same as the other claimed attributes.
• Deletes the flag definition.
• Removes the spec context that only covered the flag-off path.

Uses Changelog: other, which Danger requires when a feature flag is removed. The change only affects GitLab.com and isn't user-facing.

References

• Rollout issue: #606593
• Similar cleanup: !251635 (merged)