net: remove unused netdev_unregistering()

Currently, use dev->reg_state == NETREG_UNREGISTERING to check the status
which is NETREG_UNREGISTERING, rather than using netdev_unregistering.
Also, A helper function which is netdev_unregistering on nedevice.h is no
longer used. Thus, netdev_unregistering removes from netdevice.h.
Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -5101,11 +5101,6 @@ static inline const char *netdev_name(const struct net_device *dev)
 	return dev->name;
 }
 
-static inline bool netdev_unregistering(const struct net_device *dev)
-{
-	return dev->reg_state == NETREG_UNREGISTERING;
-}
-
 static inline const char *netdev_reg_state(const struct net_device *dev)
 {
 	switch (dev->reg_state) {