[IPV6] Remove three method declarations in include/net/ndisc.h.

This patch removes two unused method declarations in
include/net/ndisc.h: ndisc_forwarding_on(void) and
ndisc_forwarding_off(void);

Also igmp6_cleanup(void) appears twice in this header, so one
igmp6_cleanup(void) declaration is removed.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>