refactor(app): skip redundant newPayload for synced values

[mpoke]

Summary

• Remove the newPayloadV4 call from ProcessSyncedValue handler during sync/catch-up
• Synced values already have a certificate proving 2/3+ validators accepted them, making EL validation redundant
• The newPayload call in on_decided() remains, where it is required to import the block into Reth's tree state before
  forkchoiceUpdated can set it as canonical head

Context

When a node catches up via Malachite's ValueSync protocol, each synced value was
being validated with newPayloadV4 in ProcessSyncedValue. This is unnecessary
because Malachite has already verified the commit certificate, proving the value
was accepted by 2/3+ of the validator set.

The newPayload call cannot be eliminated entirely — it is the only way to import
blocks into Reth's tree state (tree_state.blocks_by_hash) before forkchoiceUpdated
can set them as canonical head. Without it, Reth returns SYNCING. This import
happens in on_decided() where it was always needed.

This change makes the intent clearer: ProcessSyncedValue trusts the certificate,
on_decided() handles the Reth integration.