block: remove unused header

Dax related code already removed from this file.
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jianpeng Ma <jianpeng.ma@intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/block_dev.c

@@ -19,7 +19,6 @@
 #include <linux/module.h>
 #include <linux/blkpg.h>
 #include <linux/magic.h>
-#include <linux/dax.h>
 #include <linux/buffer_head.h>
 #include <linux/swap.h>
 #include <linux/pagevec.h>