net: remove dead code for add/del multiple

These function to manipulate multiple addresses are not used anywhere
in current net-next tree. Some out of tree code maybe using these but
too bad; they should submit their code upstream..

Also, make __hw_addr_flush local since only used by dev_addr_lists.c

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>