ip6: retire in6_selectroute_fib() as promised 8 years ago

In r231852 I added in6_selectroute_fib() as a compat function with the
fibnum as an extra argument compared to in6_selectroute() to keep the
KPI stable.
Way too late retire this function again and add the fib to in6_selectroute()
which also only has a single consumer now and was an orphan function before.