ddl: don't use txn_last_stmt on _cluster commit/rollback

When we implement transactional DDL, txn_last_stmt won't necessarily
point to the right statement on commit or rollback so we must avoid
using it.

Note, replicas are still registered/unregisterd in the on_commit
trigger, but that's okay, as we don't really need _cluster space to
be in sync with the replica set.