unit: drop console-shell.service (#4298) (#4325)

console-shell.service was supposed to be useful for normal clean boots
(i.e. multi-user.target or so), as a replacement for logind/getty@.service for
simpler use cases.

But due to the lack of documentation and sanity check one can easily be
confused and enable this service in // with getty@.service.

In this case we end up with both services sharing the same tty which ends up in
strange results.

Even worse, console-shell.service might be failing while getty@.service tries
to acquire the terminal which ends up in the system to poweroff since
console-shell.service uses:

  "ExecStopPost=-/usr/bin/systemctl poweroff".

Another issue: this service doesn't work well if plymouth is also used since it
lets the splash screen program run and mess the tty (at least a "plymouth quit"
is missing).

So let's kill it for now.