It appears that live feature has been buggy regarding user
groups: its 30-users script would create a predefined account
with fixed supplementry groups list, and even if deflogin feature
got used too it would fail to add any groups to already existing
account since its useradd(8) call would fail.

Let's drop this duplication which has been long overdue anyways.

Thanks dd@ for both reporting the problem and carrying out
initial investigation.