chore: remove dead code in stacks-signer and stacks-node

[simone-stacks]

Two #[allow(dead_code)]-suppressed items with empty closed callgraphs:

• RawConfigFile::load_from_file — orphaned since 2023; all callers use GlobalConfig::load_from_file.
• LastCommit::start_block_hash — write-only field since Jul 2024; removing it also drops a dead .block_hash() computation at the sole call site.

Both items are workspace-internal. Result of a first smoke-test of the quality-agent.

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