[IPV6] Minor clenup: remove two unused definitions in net/ip6_route.h

Remove IP6_RT_PRIO_FW and IP6_RT_FLOW_MASK definitions in
include/net/ip6_route.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>