Staking-Async Chill stakers should not have a score

[kianenigma]

That can re-instate them in the bags-list pallet

Identified by https://github.com/paritytech-secops/srlabs_findings/issues/559

While no severe consequence, this bug could cause non-validator and non-nominator stakers to retain a spot in the bags-list pallet, preventing other legit nominators/validators from taking their place.

Note that previously, this was not a possibility, because staking would always issue a T::VoterList::on_remove when someone chills, ensuring they are removed from the list. Moreover, an older version of pallet_bags_list::Pallet::rebag didn't allow new nodes to be added, only the score of existing nodes to be adjusted.

But, in recent versions of bags-list, we added a Lock ability that would block any changes to the bags list (during the election snapshot phase). This also had us update the rebag transaction to add or remove nodes from the list, which opened the door to this issue.

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/18223885035
Failed job name: test-linux-stable