Fix merged-mining parent timestamp trust boundary

[MoneroOcean]

Motivation

• The merged-mining path allowed an untrusted child block template to overwrite the parent block header timestamp, which can produce consensus-invalid parent jobs and waste miner hashrate.

Description

• Remove the child-controlled timestamp overwrite in fillExtraMM so the parent template's consensus-relevant timestamp is no longer mutated by the child template while leaving the merged-mining extra-tag insertion intact.

Testing

• Inspected src/main.cc before and after the change and validated the offending lines were removed with sed/nl and git diff, and committed the change, all of which succeeded.

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Codex Task