Remove invalid splice debug assertion in funding_tx_constructed

We incorrectly assumed that both commitments must be at the same height,
but this is certainly possible whenever updates are proposed in both
directions at the same time.

Backport of a6ed667

Author: wpaulino

lightning/src/ln/channel.rs

@@ -6213,12 +6213,7 @@ where
 	{
 		funding.channel_transaction_parameters.funding_outpoint = Some(funding_outpoint);
 
-		if is_splice {
-			debug_assert_eq!(
-				holder_commitment_transaction_number,
-				self.counterparty_next_commitment_transaction_number,
-			);
-		} else {
+		if !is_splice {
 			self.assert_no_commitment_advancement(holder_commitment_transaction_number, "initial commitment_signed");
 			self.channel_state = ChannelState::FundingNegotiated(FundingNegotiatedFlags::new());
 		}