MDEV-275: SHOW EXPLAIN: server crashes in JOIN::print_explain with IN subquery and aggregate function
- Don't try to produce plans after JOIN::cleanup() has been called, because:
   = JOIN::cleanup leaves data structures in partially-cleaned state
   = Walking them is hazardous (see this bug), and has funny effects
     (See previous commits, "Using join cache" may or may not be shown)
   = Changing data structures to be persisted may cause unwanted side effects
- The consequence is that SHOW EXPLAIN will show "Query plan already deleted" when e.g. 
  reading data after filesort.