Clean up outdated HMAC input constant names from comments

The reserved values [5; 16], [7; 16], and [8; 16] are already flagged
as unsafe to reuse. Listing their old names from pre-LDK v0.3 days
doesn't add value anymore and risks misleading anyone into thinking
they’re still meaningful.

Author: shaavan

lightning/src/offers/signer.rs

@@ -42,11 +42,6 @@ const WITH_ENCRYPTED_PAYMENT_ID_HMAC_INPUT: &[u8; 16] = &[4; 16];
 // authentication.
 // Their corresponding values (`[5; 16]`, `[7; 16]` and `[8; 16]`) are now reserved and must not
 // be reused to ensure type confusion attacks are impossible.
-//
-// Reserved HMAC_INPUT values — do not reuse:
-//
-// const OFFER_PAYMENT_ID_HMAC_INPUT: &[u8; 16] = &[5; 16];
-// const PAYMENT_HASH_HMAC_INPUT: &[u8; 16] = &[7; 16];
 
 /// Message metadata which possibly is derived from [`MetadataMaterial`] such that it can be
 /// verified.