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.