system/skeleton: remove spurious group 'nobody'

We define the two groups 'nobody' and 'nogroup' in our skeleton, but
they have the same gid, which is not valid.

This breaks the mkuser script, as noticed by Thomas.

Anyway, the user 'nobody' belongs to the group 'nogroup' in any sane
distribution.

So, just remove the spurious 'nobody' group.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>