Remove unnecessary next_funding check

During channel_reestablish handling, next_local_commitment_number can
never be zero. Remove the unnecessary check conditioning on next_funding
not being set. Additionally, remove the spec requirement in the comment
that follows since it does not exist.

Author: jkczyz

lightning/src/ln/channel.rs

@@ -9157,11 +9157,10 @@ where
 			return Err(ChannelError::close("Peer sent a loose channel_reestablish not after reconnect".to_owned()));
 		}
 
-		if msg.next_local_commitment_number >= INITIAL_COMMITMENT_NUMBER || msg.next_remote_commitment_number >= INITIAL_COMMITMENT_NUMBER ||
-			(msg.next_local_commitment_number == 0 && msg.next_funding.is_none()) {
-			// Note: This also covers the following case in the V2 channel establishment specification:
-			//   if `next_funding` is not set, and `next_commitment_number` is zero:
-			//     MUST immediately fail the channel and broadcast any relevant latest commitment transaction.
+		if msg.next_local_commitment_number == 0
+			|| msg.next_local_commitment_number >= INITIAL_COMMITMENT_NUMBER
+			|| msg.next_remote_commitment_number >= INITIAL_COMMITMENT_NUMBER
+		{
 			return Err(ChannelError::close("Peer sent an invalid channel_reestablish to force close in a non-standard way".to_owned()));
 		}