libceph: nuke time_sub()

Unused since ceph got merged into mainline I guess.
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Alex Elder <elder@linaro.org>

include/linux/ceph/libceph.h

@@ -93,15 +93,6 @@ enum {
 	CEPH_MOUNT_SHUTDOWN,
 };
 
-/*
- * subtract jiffies
- */
-static inline unsigned long time_sub(unsigned long a, unsigned long b)
-{
-	BUG_ON(time_after(b, a));
-	return (long)a - (long)b;
-}
-
 struct ceph_mds_client;
 
 /*