ipv6: Remove optimistic DAD flag test in ipv6_add_addr()

The route we have here is for the address being added to the interface,
ie. for input packet processing.

Therefore using that route to determine whether an output nexthop gateway
is known and resolved doesn't make any sense.

So, simply remove this test, it never triggered anyways.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-By: Neil Horman <nhorman@tuxdriver.com>