Remove useless nd6_init_done

Remove useless nd6_init_done

Only ip6_init() calls nd6_init(), exactly once, just like it calls
frag6_init() which on the other hand does not have some fra6_init_done to
guard against itself.

Like all other domains, ip6_init() is called in domaininit(), early in the
kernel's main().

This variable was probably never useful and stems from nd6.c r1.1:
    bring in KAME IPv6 code, dated 19991208.

OK mvs