Remove "proto_version" from deserialize_candidate

[SpyCheese]

proto_version was passed there for consensus reasons and is not needed anymore. Removing this simplifies code.

[github-actions]

Findings

• Medium – Backward compatibility risk: collated data is now always serialized/deserialized with BOC mode 2, dropping the proto-version switch that previously used mode 31 for proto_version < 5 (validator/impl/collator.cpp:6414, validator-session/candidate-serializer.cpp:136-138, validator/collator-node/utils.cpp:66-83). If any nodes still run with proto_version < 5 or have stored candidates hashed with the old mode, mixed-version collation will produce different collated-data bytes/hashes and fail to interoperate. Please confirm the minimum supported proto_version or gate this behavior with a protocol bump.