`MAC`+`PUBKEY`+`SIGNATURE` types are dead weight

[plebhash]

every data type is defined so it can be used in message fields

MAC+PUBKEY+SIGNATURE are cryptographic primitives and not used in any message field, so there's no point in specifying it as a data type

[Shourya742]

We do have other datatypes like Signature and PubKey, which are defined in Specs and in our implementation but are never used. Then we should also remove these from both our implementation and specs.