Code review/cleanup of sub subselect_uniquesubquery_engine


sql/ha_myisam.cc:
  Removed unnecessary casts
sql/item_subselect.cc:
  Safety fix (reset engine)
  Code review/cleanup of sub subselect_uniquesubquery_engine
  Remove 'end_exec'. (Should be done on query end, not in the middle)
sql/item_subselect.h:
  Removed end_exec();
sql/mysqld.cc:
  Don't print warnings about 'setrlimit' if --warnings is not given