Method clean up:

- removed unused methods from MYSQL_STMT handle,
  (left over from PHP's mysqlnd)
- Added execute_generate_request method: This will allow
  Connector/Python to prefill the execute buffer without
  numerous GIL acquire/release calls.

include/mysql.h

@@ -874,6 +874,7 @@ struct st_mariadb_methods {
   void (*invalidate_stmts)(MYSQL *mysql, const char *function_name);
   struct st_mariadb_api *api;
   int (*db_read_execute_response)(MYSQL_STMT *stmt);
+  unsigned char* (*db_execute_generate_request)(MYSQL_STMT *stmt, size_t *request_len, my_bool internal);
 };
 
 /* synonyms/aliases functions */