removed punbb auth backend

Since PunBB 1.2.x is no longer current but PunBB 1.3.x is not supported by the
backend it is removed from the default distribution. The code was put up on
http://www.dokuwiki.org/auth:punbb

Note, that changes making an 1.3.x compatible version possible where pushed
earlier this day.

darcs-hash:20090114203914-7ad00-07063accb48675952792b9d06760bf9a748d5cd1.gz