swiotlb: Drop unused functions swiotlb_map_sg and swiotlb_unmap_sg

There are no users for swiotlb_map_sg or swiotlb_unmap_sg so we might as
well just drop them.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad@kernel.org>