A cleanup for MDEV-31578 DECLARE CURSOR: "Memory not freed: 280 bytes lost" on syntax error

Removing the method LEX::delete_if_not_sp_lex_in_use(). It was not really used.

Author: abarkov

sql/sql_lex.h

@@ -3429,11 +3429,7 @@ struct LEX: public Query_tables_list
  sp_head *sphead;
  sp_name *spname;
  bool sp_lex_in_use; // Keep track on lex usage in SPs for error handling
- void delete_if_not_sp_lex_in_use()
- {
- if (!sp_lex_in_use)
- delete this;
- }
+
  sp_pcontext *spcont;
 
  st_sp_chistics sp_chistics;