code cleanup


mysql-test/r/subselect.result:
  test for BUG#1668
mysql-test/t/subselect.test:
  test for BUG#1668
sql/item_subselect.cc:
  removed unused class field
  fixed min_max subquery used_tables()/const()
  fixed ALL/ANY optimisation for unions
sql/item_subselect.h:
  removed unused class field
  fixed min_max subquery used_tables()/const()
sql/sql_union.cc:
  fixed result object assignment