ipv6: don't disable interface if last ipv6 address is removed

The reason behind this change is that as soon as we delete
the last ipv6 address of an interface we also lose the
/proc/sys/net/ipv6/conf/<interface> directory. This seems to be a
usability problem for me.

I don't see any reason why we should shutdown ipv6 on that interface in
such cases.

Cc: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>