hexagon: remove the unused dma_is_consistent prototype

Signed-off-by: Christoph Hellwig <hch@lst.de>

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

@@ -37,7 +37,6 @@ static inline const struct dma_map_ops *get_arch_dma_ops(struct bus_type *bus)
 	return dma_ops;
 }
 
-extern int dma_is_consistent(struct device *dev, dma_addr_t dma_handle);
 extern void dma_cache_sync(struct device *dev, void *vaddr, size_t size,
 			   enum dma_data_direction direction);