ipv6: Remove unnecessary test

The "init_net" test in function addrconf_exit_net is introduced
in commit 44a6bd29 [Create ipv6 devconf-s for namespaces] to avoid freeing
init_net. In commit c900a800 [ipv6: fix bad free of addrconf_init_net],
function addrconf_init_net will allocate memory for every net regardless of
init_net. In this case, it is unnecessary to make "init_net" test.

CC: Hong Zhiguo <honkiko@gmail.com>
CC: Octavian Purdila <opurdila@ixiacom.com>
CC: Pavel Emelyanov <xemul@openvz.org>
CC: Cong Wang <cwang@twopensource.com>
Suggested-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Zhu Yanjun <Yanjun.Zhu@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>