nfs: remove NFS_WBACK_BUSY()

Nothing calls this macro.

Signed-off-by: Jeff Layton <[e-mail]>
Signed-off-by: Anna Schumaker <[e-mail]>

include/linux/nfs_page.h

···
 /* arbitrarily selected limit to number of mirrors */
 #define NFS_PAGEIO_DESCRIPTOR_MIRROR_MAX 16
 
-#define NFS_WBACK_BUSY(req)	(test_bit(PG_BUSY,&(req)->wb_flags))
-
 extern struct nfs_page *nfs_page_create_from_page(struct nfs_open_context *ctx,
 						  struct page *page,
 						  unsigned int pgbase,