invoices+sql: use the stored AmtPaid value instead of recalculating

Previously we'd recalculate the paid amount by summing amounts of
settled HTLCs. This approach while correct would stop the SQL migration
process as some KV invoices may have incorrectly stored paid amounts.

invoices/sql_store.go

@@ -1576,13 +1576,6 @@ func fetchInvoiceData(ctx context.Context, db SQLInvoiceQueries,
 
 	if len(htlcs) > 0 {
 		invoice.Htlcs = htlcs
-		var amountPaid lnwire.MilliSatoshi
-		for _, htlc := range htlcs {
-			if htlc.State == HtlcStateSettled {
-				amountPaid += htlc.Amt
-			}
-		}
-		invoice.AmtPaid = amountPaid
 	}
 
 	return hash, invoice, nil