MDEV-28893 Spider: remove #ifdef SPIDER_NET_HAS_THD

net has thd since 2015 in 56aa1998 for MDEV-6152

storage/spider/spd_db_mysql.cc

@@ -2032,9 +2032,7 @@ int spider_db_mbase::connect(
       connect_retry_count--;
       my_sleep((ulong) connect_retry_interval);
     } else {
-#ifdef SPIDER_NET_HAS_THD
       db_conn->net.thd = NULL;
-#endif
       if (connect_mutex)
         pthread_mutex_unlock(&spider_open_conn_mutex);
       break;

storage/spider/spd_environ.h

@@ -23,7 +23,6 @@
 #define SPIDER_HANDLER_START_BULK_INSERT_HAS_FLAGS
 
 #define SPIDER_SUPPORT_CREATE_OR_REPLACE_TABLE
-#define SPIDER_NET_HAS_THD
 
 #define HANDLER_HAS_TOP_TABLE_FIELDS
 #define HANDLER_HAS_DIRECT_UPDATE_ROWS