MDEV-37018 Remove SELECT_LEX::select_h

It is unused.

Author: mariadb-YuchenPei

sql/sql_lex.h

@@ -1150,8 +1150,6 @@ class st_select_lex: public st_select_lex_node
   TABLE_LIST *embedding;          /* table embedding to the above list   */
   table_value_constr *tvc;
 
-  /* The interface employed to execute the select query by a foreign engine */
-  select_handler *select_h;
   /* The object used to organize execution of the query by a foreign engine */
   select_handler *pushdown_select;
   List<TABLE_LIST> *join_list;    /* list for the currently parsed join  */