Drop passwordless sudo

Membership in the sudo group is ensured by vmdebootstrap's --sudo parameter.

Upgrade log:

$ sudo dpkg -i librem5-devkit-base_0.0.20pureos1+librem5ci70355.77167bc_arm64.deb 
(Reading database ... 81451 files and directories currently installed.)
Preparing to unpack librem5-devkit-base_0.0.20pureos1+librem5ci70355.77167bc_arm64.deb ...
Unpacking librem5-devkit-base (0.0.20pureos1+librem5ci70355.77167bc) over (0.0.19pureos1) ...
Removed /etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service.
Setting up librem5-devkit-base (0.0.20pureos1+librem5ci70355.77167bc) ...
Created symlink /etc/systemd/system/getty.target.wants/serial-getty@ttyGS0.service → /lib/systemd/system/serial-getty@.service.
Removing obsolete conffile /etc/sudoers.d/devkit ...

# and from now on:
purism@pureos:/tmp$ sudo -s

We trust you have received the usual lecture from the local System
Administrator. It usually boils down to these three things:

    #1) Respect the privacy of others.
    #2) Think before you type.
    #3) With great power comes great responsibility.

[sudo] password for purism: 
root@pureos:/tmp# 

I'd welcome a quick review here so we can push that out into the new images. I've set urgency high to clear the way quickly for other pending changes to this package.

Closes: OS-issues#208 (closed)