atx: remove node id field from atx

[dshulyak]

Sorry for only getting around to looking at this after it was merged, but I think we should remove the NodeID from the ATX. It was only there when we needed to maintain an identity database - now that we don't, we should derive the public key from the signature and do all validations on that (e.g. that the PoST proof validates with this key and that the key we derived from the previous ATX matches). It can (probably should) be cached in the database along with the ATX, but not included in the gossip message. This will be consistent with all our other messages.

Originally posted by @noamnelke in https://github.com/spacemeshos/go-spacemesh/issues/3198#issuecomment-1128749597