Remove Kraken (mimir-killing) functionality

The Kraken concept predates the usage of Mimir as an on-chain shared key-value store.

It has never been set so this change is safe to do unversioned.

Instead of adding it to the admin deny list it seems better to remove it entirely, as it is dangerous for anyone to set.

Thanks to @Multipartite for pointing this out as a candidate for removal 🙏