blackfin: remove sg_address()

I would have replaced it with sg_virt(), but it doesn't appear to be
used at all.
Signed-off-by: NJens Axboe <jens.axboe@oracle.com>

include/asm-blackfin/scatterlist.h

@@ -20,7 +20,6 @@ struct scatterlist {
  * returns, or alternatively stop on the first sg_dma_len(sg) which
  * is 0.
  */
-#define sg_address(sg) (page_address((sg)->page) + (sg)->offset)
 #define sg_dma_address(sg)      ((sg)->dma_address)
 #define sg_dma_len(sg)          ((sg)->length)