Fix deny(const_err) lint warnings

This lint is deny-by-default since Rust 1.51 and has recently been
made a hard error, which should ensure that we continue to get coverage,
at the very least in beta or nightly CI.

See rust-lang/rust#71800

lightning/src/ln/channelmanager.rs

@@ -949,13 +949,11 @@ pub const MIN_FINAL_CLTV_EXPIRY: u32 = HTLC_FAIL_BACK_BUFFER + 3;
 // then waiting ANTI_REORG_DELAY to be reorg-safe on the outbound HLTC and
 // failing the corresponding htlc backward, and us now seeing the last block of ANTI_REORG_DELAY before
 // LATENCY_GRACE_PERIOD_BLOCKS.
-#[deny(const_err)]
 #[allow(dead_code)]
 const CHECK_CLTV_EXPIRY_SANITY: u32 = MIN_CLTV_EXPIRY_DELTA as u32 - LATENCY_GRACE_PERIOD_BLOCKS - CLTV_CLAIM_BUFFER - ANTI_REORG_DELAY - LATENCY_GRACE_PERIOD_BLOCKS;
 
 // Check for ability of an attacker to make us fail on-chain by delaying an HTLC claim. See
 // ChannelMonitor::should_broadcast_holder_commitment_txn for a description of why this is needed.
-#[deny(const_err)]
 #[allow(dead_code)]
 const CHECK_CLTV_EXPIRY_SANITY_2: u32 = MIN_CLTV_EXPIRY_DELTA as u32 - LATENCY_GRACE_PERIOD_BLOCKS - 2*CLTV_CLAIM_BUFFER;
 

lightning/src/ln/peer_handler.rs

@@ -1981,7 +1981,6 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 	// broadcast_node_announcement panics) of the maximum-length addresses would fit in a 64KB
 	// message...
 	const HALF_MESSAGE_IS_ADDRS: u32 = ::core::u16::MAX as u32 / (NetAddress::MAX_LEN as u32 + 1) / 2;
-	#[deny(const_err)]
 	#[allow(dead_code)]
 	// ...by failing to compile if the number of addresses that would be half of a message is
 	// smaller than 100: