!1202 [sync] PR-1148: LoongArch: cleanup swiotlb bakcup buffer related code

Merge Pull Request from: @openeuler-sync-bot 
 

Origin pull request: 
https://gitee.com/openeuler/kernel/pulls/1148 
 
cleanup swiotlb bakcup buffer related code,not used now. 
 
Link:https://gitee.com/openeuler/kernel/pulls/1202

 

Reviewed-by: Jialin Zhang <zhangjialin11@huawei.com>
Signed-off-by: Jialin Zhang <zhangjialin11@huawei.com>