[IPV6] Minor cleanup: remove unused definitions in net/ip6_fib.h

This patch removes some unused definitions and one method typedef
declaration (f_pnode)
in include/net/ip6_fib.h, as they are not used in the kernel.

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