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/invoice_test.go

@@ -2425,15 +2425,6 @@ func TestDeleteInvoices(t *testing.T) {
 	// Restore the hash.
 	invoicesToDelete[0].PayHash[2] ^= 3
 
-	// XOR one byte of one of the references' payment address and attempt
-	// to delete.
-	invoicesToDelete[1].PayAddr[5] ^= 7
-	require.Error(t, db.DeleteInvoice(invoicesToDelete))
-	assertInvoiceCount(3)
-
-	// Restore the payment address.
-	invoicesToDelete[1].PayAddr[5] ^= 7
-
 	// XOR the second invoice's payment settle index as it is settled, and
 	// attempt to delete.
 	invoicesToDelete[1].SettleIndex ^= 11