Stop writing old user_payment_id field not read since 0.0.103

Since we're breaking downgrade compatibility to LDK version 0.0.103
and before, we might as well go ahead and remove other code we have
for compatibility.

Author: TheBlueMatt

lightning/src/events/mod.rs

@@ -855,7 +855,7 @@ impl Writeable for Event {
 					(3, via_channel_id, option),
 					(4, amount_msat, required),
 					(5, via_user_channel_id, option),
-					(6, 0u64, required), // user_payment_id required for compatibility with 0.0.103 and earlier
+					// Type 6 was `user_payment_id` on 0.0.103 and earlier
 					(7, claim_deadline, option),
 					(8, payment_preimage, option),
 					(9, onion_fields, option),
@@ -1059,7 +1059,7 @@ impl MaybeReadable for Event {
 					let mut payment_secret = None;
 					let mut amount_msat = 0;
 					let mut receiver_node_id = None;
-					let mut _user_payment_id = None::<u64>; // For compatibility with 0.0.103 and earlier
+					let mut _user_payment_id = None::<u64>; // Used in 0.0.103 and earlier, no longer written in 0.0.116+.
 					let mut via_channel_id = None;
 					let mut claim_deadline = None;
 					let mut via_user_channel_id = None;

pending_changelog/no-legacy-payments.txt

@@ -1,4 +1,6 @@
  * Legacy inbound payment creation has been removed, thus there is no way to
    create a pending inbound payment which will still be claimable on LDK
    0.0.103 or earlier. Support for claiming such payments is still retained,
-   however is likely to be removed in the next release.
+   however is likely to be removed in the next release (#2351).
+ * Some fields required in 0.0.103 and earlier are no longer written, thus
+   deserializing objects written in 0.0.116 with 0.0.103 may now fail (#2351).