etc/group: admin group causes pw(1) error

I'm not entirely sure if deletion or reverting to id 101 is the better option. Feel free to discuss.


The issue was introduced in commit 128f6a3e in 2005: the admin
user was mapped to wheel and group admin was set to gid 0,
which clashes with the wheel group:

    php: rc.bootup: The command '/usr/sbin/pw userdel -n 'admin''
    returned exit code '1', the output was 'pw: entry inconsistent
    pw: gr_copy(): Invalid argument'

Delete the admin group since it is no longer used.