powerpc: remove unused flush_write_buffers definition

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

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

@@ -107,9 +107,6 @@ static inline void set_dma_offset(struct device *dev, dma_addr_t off)
 		dev->archdata.dma_offset = off;
 }
 
-/* this will be removed soon */
-#define flush_write_buffers()
-
 #define HAVE_ARCH_DMA_SET_MASK 1
 extern int dma_set_mask(struct device *dev, u64 dma_mask);