invoices: do not fail DeleteInvoice if payment addr isn't indexed

This commit relaxes DeleteInvoice failure cases by removing the
requirement that the invoice needs to be indexed by the payment address.
Since payment address index was introduced with an empty migration it
is possible that users have old invoices which were never added to
this index causing invoice garbage collection to get stuck.

channeldb/invoices.go

@@ -2305,15 +2305,19 @@ func (d *DB) DeleteInvoice(invoicesToDelete []InvoiceDeleteRef) error {
 				// fetched invoice key matches the one in the
 				// payment address index.
 				key := payAddrIndex.Get(ref.PayAddr[:])
-				if !bytes.Equal(key, invoiceKey) {
-					return fmt.Errorf("unknown invoice " +
-						"in pay addr index")
-				}
-
-				// Delete from the payment address index.
-				err := payAddrIndex.Delete(ref.PayAddr[:])
-				if err != nil {
-					return err
+				if bytes.Equal(key, invoiceKey) {
+					// Delete from the payment address index.
+					// Note that since the payment address
+					// index has been introduced with an
+					// empty migration it may be possible
+					// that the index doesn't have an entry
+					// for this invoice.
+					// ref: https://github.com/lightningnetwork/lnd/pull/4285/commits/cbf71b5452fa1d3036a43309e490787c5f7f08dc#r426368127
+					if err := payAddrIndex.Delete(
+						ref.PayAddr[:],
+					); err != nil {
+						return err
+					}
 				}
 			}