sulogin-shell: remove ineffective job mode option from `systemctl isolate` (#6627)

`systemctl default` uses job mode `isolate` (see `action_table`).
The job mode option is ignored.

Note that exiting the emergency shell service by using e.g.
`systemctl isolate multi-user` or `systemctl start multi-user.target`
already kills `emergency.service`.  There's only a potential conflict
between your command and the command in systemd-sulogin-shell if you run
something like `systemctl start --no-block multi-user.target; exit`.
Which is nothing like what we told them to do :).