ipv6: eliminate ndisc_ops_is_useropt()

ipv6: eliminate ndisc_ops_is_useropt()

as it doesn't seem to offer anything of value.

There's only 1 trivial user:
  int lowpan_ndisc_is_useropt(u8 nd_opt_type) {
    return nd_opt_type == ND_OPT_6CO;
  }

but there's no harm to always treating that as
a useropt...

Cc: David Ahern <dsahern@kernel.org>
Cc: YOSHIFUJI Hideaki / 吉藤英明 <yoshfuji@linux-ipv6.org>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Link: https://patch.msgid.link/20240730003010.156977-1-maze@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>