feat: remove accounts on confirmed undelegation

[GabrielePicco]

Request

Removing accounts upon confirmed undelegation eliminates the possibility of stuck accounts caused by a missed delegation event, where the account already exists in the ER. Ideally, events are never missed, but this change removes the user-blocking behavior.

First attempt in this PR: #763

[thlorenz]

The PR was closed in favor of reliable updates, so could we close this as well @GabrielePicco ?

Removal would have to be performed as a proper transaction which then also checks that the account is still undelegated in the bank.
Otherwise we could remove it while a transaction is already scheduled that tries to read it.
So not as trivial as just removing directly from the bank.