AI: unrestrict finalize_unstake
As the operation only unblocks tez without changing anything else, there is no reason to restrict this operation to the sender. So, we can:
- ignore the sender, the sender is the payer, but the operation will be performed on the destination
- hence allow it from internal operations
- make sure the gas cost of finalize is not counted if there is nothing to do (especially when done automatically with unstake)
Actually I was lying when I said it doesn't change anything else. If a staker changes delegate, it will change who the delegate tez will count towards. The old delegate when not finalized, the new one when finalized. It will be an incentive for delegates to apply this operation on accounts that just switched delegate.