Remove redundant codes purging IPv6 addresses

[Ryota Ozaki <ozaki-r%netbsd.org@localhost>]

Hi,

As dyoung suggests in comments, in6_purgeif and
in6_ifdetach have some redundant codes.

- in6_purgeif calls if_purgeaddrs and in6_ifdetach,
  but if_purgeaddrs is also called in in6_ifdetach.
  I think we can remove either of them.
- in6_ifdetach has very similar codes with if_purgeaddrs.
  I think we can remove the codes from in6_ifdetach.

Such redundant codes makes MP-ification difficult
meaninglessly. I really hope removing them.

Here is a patch that removes the redundant codes:
  http://www.netbsd.org/~ozaki-r/remove-redundant-purging-addresses.diff

I confirmed that the change doesn't break ATF tests
and a debug helper function for ifaddr (added by me
recently) shows there remain no address after
if_detach.

Any objections?

  ozaki-r