Drop useless doc reference to pre-zero-HTLC-fee anchor channels

These aren't really used anywhere and were only live very briefly,
so there's not really any point in informing our users that we
don't support them. If anything, it will lead to confusion as the
zero-HTLC-fee channel type is generally referred to simply as
"anchor channels".

Author: TheBlueMatt

lightning/src/util/config.rs

@@ -176,16 +176,11 @@ pub struct ChannelHandshakeConfig {
 	/// counterparties that do not support the `anchors_zero_fee_htlc_tx` option; we will simply
 	/// fall back to a `static_remote_key` channel.
 	///
-	/// LDK will not support the legacy `option_anchors` commitment version due to a discovered
-	/// vulnerability after its deployment. For more context, see the [`SIGHASH_SINGLE + update_fee
-	/// Considered Harmful`] mailing list post.
-	///
 	/// Default value: `false` (This value is likely to change to `true` in the future.)
 	///
 	/// [`ChannelManager`]: crate::ln::channelmanager::ChannelManager
 	/// [`ChannelManager::accept_inbound_channel`]: crate::ln::channelmanager::ChannelManager::accept_inbound_channel
 	/// [`DecodeError::InvalidValue`]: crate::ln::msgs::DecodeError::InvalidValue
-	/// [`SIGHASH_SINGLE + update_fee Considered Harmful`]: https://lists.linuxfoundation.org/pipermail/lightning-dev/2020-September/002796.html
 	pub negotiate_anchors_zero_fee_htlc_tx: bool,
 
 	/// The maximum number of HTLCs in-flight from our counterparty towards us at the same time.