net: Kill NETEVENT_PMTU_UPDATE.

Nobody actually does anything in response to the event,
so just kill it off.

Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/netevent.h

@@ -21,7 +21,6 @@ struct netevent_redirect {
 
 enum netevent_notif_type {
 	NETEVENT_NEIGH_UPDATE = 1, /* arg is struct neighbour ptr */
-	NETEVENT_PMTU_UPDATE,	   /* arg is struct dst_entry ptr */
 	NETEVENT_REDIRECT,	   /* arg is struct netevent_redirect ptr */
 };