manager: remove fallback for user/exit.target

manager: remove fallback for user/exit.target

The comment here was misleading: the job can fail to enqueue for reasons
other than the target not existing.

The fallback caused an error to be logged, and dates back to when the
"user" directory was named "session".  units/session/exit.target was added
later the same year.

This is consistent with the documentation (man systemd), and the handling
of similar signals.  It's also consistent with `systemctl exit`, which is
what most people would expect.