Remove next_funding_txid tlv from channel read/write

We want to remove this before release so that we can work on a way to
not persist this but rather get it from other persisted data and just
free up the TLV.

Note that the "added in 0.0.124" comment was incorrect as it was
actually added in lightningdevkit#3137 but the comment was stale so it's safe to remove.

lightning/src/ln/channel.rs

@@ -9507,7 +9507,6 @@ impl<SP: Deref> Writeable for Channel<SP> where SP::Target: SignerProvider {
 			(49, self.context.local_initiated_shutdown, option), // Added in 0.0.122
 			(51, is_manual_broadcast, option), // Added in 0.0.124
 			(53, funding_tx_broadcast_safe_event_emitted, option), // Added in 0.0.124
-			(55, self.context.next_funding_txid, option) // Added in 0.1.0
 		});
 
 		Ok(())
@@ -9797,7 +9796,6 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 		let mut channel_pending_event_emitted = None;
 		let mut channel_ready_event_emitted = None;
 		let mut funding_tx_broadcast_safe_event_emitted = None;
-		let mut next_funding_txid = funding_transaction.as_ref().map(|tx| tx.compute_txid());
 
 		let mut user_id_high_opt: Option<u64> = None;
 		let mut channel_keys_id: Option<[u8; 32]> = None;
@@ -9858,7 +9856,6 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 			(49, local_initiated_shutdown, option),
 			(51, is_manual_broadcast, option),
 			(53, funding_tx_broadcast_safe_event_emitted, option),
-			(55, next_funding_txid, option) // Added in 0.0.125
 		});
 
 		let (channel_keys_id, holder_signer) = if let Some(channel_keys_id) = channel_keys_id {
@@ -10119,10 +10116,13 @@ impl<'a, 'b, 'c, ES: Deref, SP: Deref> ReadableArgs<(&'a ES, &'b SP, u32, &'c Ch
 
 				blocked_monitor_updates: blocked_monitor_updates.unwrap(),
 				is_manual_broadcast: is_manual_broadcast.unwrap_or(false),
+				// TODO(dual_funding): Instead of getting this from persisted value, figure it out based on the
+				// funding transaction and other channel state.
+				//
 				// If we've sent `commtiment_signed` for an interactively constructed transaction
 				// during a signing session, but have not received `tx_signatures` we MUST set `next_funding_txid`
 				// to the txid of that interactive transaction, else we MUST NOT set it.
-				next_funding_txid,
+				next_funding_txid: None,
 			},
 			interactive_tx_signing_session: None,
 		})