net: Kill support for multiple hh_cache entries per neighbour

This never, ever, happens.

Neighbour entries are always tied to one address family, and therefore
one set of dst_ops, and therefore one dst_ops->protocol "hh_type"
value.

This capability was blindly imported by Alexey Kuznetsov when he wrote
the neighbour layer.

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