Remove redundant payment preimag hashing in HTLC claim pipeline

Currently, when we receive an HTLC claim from a peer, we first hash
the preimage they gave us before removing the HTLC, then
immediately pass the preimage to the inbound channel and hash the
preimage again before removing the HTLC and sending our peer an
`update_fulfill_htlc`. This second hash is actually only asserted
on, never used in any meaningful way as we have the htlc data
present in the same code.

Here we simply drop this second hash and move it into a
`debug_assert`.

Author: TheBlueMatt

lightning/src/ln/channel.rs

@@ -2202,8 +2202,6 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		}
 		assert_eq!(self.context.channel_state & ChannelState::ShutdownComplete as u32, 0);
 
-		let payment_hash_calc = PaymentHash(Sha256::hash(&payment_preimage_arg.0[..]).into_inner());
-
 		// ChannelManager may generate duplicate claims/fails due to HTLC update events from
 		// on-chain ChannelsMonitors during block rescan. Ideally we'd figure out a way to drop
 		// these, but for now we just have to treat them as normal.
@@ -2212,7 +2210,7 @@ impl<Signer: WriteableEcdsaChannelSigner> Channel<Signer> {
 		let mut htlc_value_msat = 0;
 		for (idx, htlc) in self.context.pending_inbound_htlcs.iter().enumerate() {
 			if htlc.htlc_id == htlc_id_arg {
-				assert_eq!(htlc.payment_hash, payment_hash_calc);
+				debug_assert_eq!(htlc.payment_hash, PaymentHash(Sha256::hash(&payment_preimage_arg.0[..]).into_inner()));
 				log_debug!(logger, "Claiming inbound HTLC id {} with payment hash {} with preimage {}",
 					htlc.htlc_id, htlc.payment_hash, payment_preimage_arg);
 				match htlc.state {