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),