neigh: Simply destroy handling wrt. hh_cache.

neigh: Simply destroy handling wrt. hh_cache.

Now that hh_cache entries are embedded inside of neighbour
entries, their lifetimes and accesses are now synchronous
to that of the encompassing neighbour object.

Therefore we don't need to hook up the blackhole op to
hh_output on destroy.

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