net/ipv6: Remove compare of fib6_idev from rt6_duplicate_nexthop

After 4832c30d5458 ("net: ipv6: put host and anycast routes on device
with address") the comparison of idev does not add value since it
correlates to the nexthop device which is already compared. Remove
the idev comparison.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>