Remove ECChain value from justification entirely

[masih]

Why does GPBFT justification format bother with including the chain at all? It seems for any given message we can always infer: 1) whether it should be justified, and 2) what its justified chain should be.

At the wire level communication at least there is no need at all for having it.

Its removal will also simplify the FIP.

[BigLep]

2025-03-19 standup: agreed not to take on before F3 activation so as to avoid introducing unnecessary code changes after the code freeze. This can get picked up post activation. I have adjusted milestone accordingly.

[BigLep]

Per internal 2025-03-28 slack conversation, it wasn't deemed critical to make it an immediate followup/cleanup after F3 activation. As a result, I have reassigned its milestone.

[BigLep]

2025-05-02 conversation: this will need FIP updates and testing. This could drag on for more than a eek. We're going to defer this to a later point.