Previously-authentication fields in contexts can be reused

We recently switched to `ReceiveAuthKey`-based blinded path
authentication, removing various fields used to authenticate
blinded paths from contexts. In doing so, we left a comment arguing
that such fields should not have their TLV IDs reused, however
there's no reason for that restriction. If we reuse old blinded
paths with those fields set they'll fail the new authentication
checks anyway, so the fields should never end up being seen in
blinded paths we'd accept.

This simply updates the comments.

Author: TheBlueMatt

lightning/src/blinded_path/message.rs

@@ -543,27 +543,20 @@ impl_writeable_tlv_based_enum!(MessageContext,
 	{3, DNSResolver} => (),
 );
 
-// NOTE:
-// Several TLV fields (`nonce`, `hmac`, etc.) were removed in LDK v0.2
-// following the introduction of `ReceiveAuthKey`-based authentication for
-// inbound `BlindedMessagePath`s. These fields are now commented out and
-// their `type` values must not be reused unless support for LDK v0.2
-// and earlier is fully dropped.
-//
-// For context-specific removals, see the commented-out fields within each enum variant.
+// Note: Several TLV fields (`nonce`, `hmac`, etc.) were removed in LDK v0.2 following the
+// introduction of `ReceiveAuthKey`-based authentication for inbound `BlindedMessagePath`s. Because
+// we do not support receiving to those contexts anymore (they will fail the `ReceiveAuthKey`-based
+// authentication checks), we can reuse those fields here.
 impl_writeable_tlv_based_enum!(OffersContext,
 	(0, InvoiceRequest) => {
 		(0, nonce, required),
 	},
 	(1, OutboundPayment) => {
 		(0, payment_id, required),
 		(1, nonce, required),
-		// Removed: (2, hmac, option)
 	},
 	(2, InboundPayment) => {
 		(0, payment_hash, required),
-		// Removed: (1, nonce, required),
-		// Removed: (2, hmac, required)
 	},
 	(3, StaticInvoiceRequested) => {
 		(0, recipient_id, required),
@@ -575,12 +568,8 @@ impl_writeable_tlv_based_enum!(OffersContext,
 impl_writeable_tlv_based_enum!(AsyncPaymentsContext,
 	(0, OutboundPayment) => {
 		(0, payment_id, required),
-		// Removed: (2, nonce, required),
-		// Removed: (4, hmac, required),
 	},
 	(1, InboundPayment) => {
-		// Removed: (0, nonce, required),
-		// Removed: (2, hmac, required),
 		(4, path_absolute_expiry, required),
 	},
 	(2, OfferPaths) => {