Revert "LoongArch: Add swiotlb backups buffer"

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I7DWYY

--------------------------------

This reverts commit f728ed91.
Signed-off-by: NJuxin Gao <gaojuxin@loongson.cn>
Signed-off-by: NHongchen Zhang <zhanghongchen@loongson.cn>

arch/loongarch/include/asm/dma-mapping.h

-/* SPDX-License-Identifier: GPL-2.0 */
-/*
- * Copyright (C) 2020 Loongson Technology Corporation Limited
- */
-#ifndef _ASM_DMA_MAPPING_H
-#define _ASM_DMA_MAPPING_H
-
-extern const struct dma_map_ops loongson_dma_ops;
-extern bool swiotlb_need_fix;
-extern phys_addr_t io_tlb_start;
-
-static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
-{
-	if (swiotlb_need_fix)
-		return &loongson_dma_ops;
-	else
-		return NULL;
-}
-
-#endif /* _ASM_DMA_MAPPING_H */