[TASK] Remove superfluous/extra encryption in keygen

[drewstone]

Issue summary
When we originally modified the Zengo mp-ecdsa, we added additional encryption to P2P messages because we were originally broadcasting/gossiping all messages over our network,

Some old commits which added this extra encryption are:

• tangle-network/multi-party-ecdsa@a4895af
• tangle-network/multi-party-ecdsa@033d799
• tangle-network/multi-party-ecdsa@cd894b6
• tangle-network/multi-party-ecdsa@1ef80e1

I think we should remove this and make more explicit how this type of message can be encrypted before the P2P messages hit the wire.

[ivokub]

Yup, in CGGMP the assumption is that the channels are authenticated between parties and broadcasts are reliable. Our approach is to use Noise protocol on a higher level which provides authenticated encryption between all parties and allows to use untrusted proxy for relaying messages. See https://github.com/mpc-sdk/framework.