Correct TRASH() macro usage

TRASH was mapped to TRASH_FREE and was supposed to be used for memory
that should not be accessed anymore, while TRASH_ALLOC() is to be
used for uninitialized but to-be-used memory.

But sometimes TRASH() was used in the latter sense.

Remove TRASH() macro, always use explicit TRASH_ALLOC() or TRASH_FREE().

sql/sql_union.cc

@@ -935,22 +935,6 @@ bool st_select_lex_unit::cleanup()
 void st_select_lex_unit::reinit_exec_mechanism()
 {
   prepared= optimized= executed= 0;
-#ifndef DBUG_OFF
-  if (is_union())
-  {
-    List_iterator_fast<Item> it(item_list);
-    Item *field;
-    while ((field= it++))
-    {
-      /*
-	we can't cleanup here, because it broke link to temporary table field,
-	but have to drop fixed flag to allow next fix_field of this field
-	during re-executing
-      */
-      field->fixed= 0;
-    }
-  }
-#endif
 }