dma-mapping: remove the dma_dummy_ops export

dma_dummy_ops is only used by the ACPI code, which can't be modular.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>

kernel/dma/dummy.c

@@ -36,4 +36,3 @@ const struct dma_map_ops dma_dummy_ops = {
 	.map_sg                 = dma_dummy_map_sg,
 	.dma_supported          = dma_dummy_supported,
 };
-EXPORT_SYMBOL(dma_dummy_ops);