- Moving opt_union_order_or_limit inside union_opt,
  as it's not used in other places any more.
- Changing union_opt to have no type.
  Earlier (before all MDEV-8909 dependency tasks) it had the <num> type,
  and it's return value was used to generate errors.
  Now union_opt does not need a return value because the grammar
  disallows ORDER and LIMIT clauses in wrong context.