This is actually what the autoclean plugin wants, especially since
you can't otherwise delete a payment which has failed then succeeded.
But insist on neither or both being specified, at least for now.
Changelog-Added: JSON-RPC: `delpay` takes optional `groupid` and `partid` parameters to specify exactly what payment to delete.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
