Clean up unused serialization code

[johnathan79717]

Serializations in bb comes in the following forms:

• MSGPACK_FIELDS macro - Defined in msgpack.hpp:121-125 - generates a simple msgpack() method for declarative serialization
• Custom msgpack_pack/msgpack_unpack methods - Used when special logic is needed (e.g., Montgomery form conversion, point-at-infinity handling)
• Free read/write functions - Used for binary serialization (separate from msgpack)

Some of them might be defined on some classes but unused and can be cleaned up.

cc @ledwards2225

[johnathan79717]

Claude suggests these to start with:

Type	Location	MSGPACK_FIELDS	Actually Serialized	Status	Recommendation
RelationParameters	relations/relation_parameters.hpp:98	YES	NO (unused)	Dead Code	SAFE TO REMOVE
GoblinStdlibProof	goblin/types.hpp:54	YES	NO (circuit use only)	Dead Code	SAFE TO REMOVE
PublicInputsAndProof	honk/proof_system/types/proof.hpp:20	YES	NO (never used)	Dead Code	SAFE TO REMOVE
VerifierInstance	ultra_honk/verifier_instance.hpp:113	YES	NO (unused)	Dead Code	SAFE TO REMOVE

[ludamad]

It seems like you have made good progress. in terms of fully removing the 'flat' serialization (common/ utilities) we probably could only really pare it down as we will still serialize at least proofs using it. The other thing I'll mention is to definitely check any serialization code in generate from 1.5+ years ago

[johnathan79717]

From Adam: maybe if it is just a few places better is just to write out the read/write functions and have them call a helper that works for any msgpack object, and at that point, document why those exist as it's clearly hard to reconstruct later (edited), so we'd just make those helpers msgpack_write_impl/msgpack_read_impl or something and just write out the read/write so it's clear they exist.

[johnathan79717]

AztecProtocol/aztec-packages#19060 removes some unused field declarations.