run: reject waiting for remain-after-exit services

run: reject waiting for remain-after-exit services

Reproducer:
  unit=run-wait-rae-$(date +%s)
  sudo timeout 3s systemd-run --wait --remain-after-exit \
      --unit="$unit" /bin/true
  echo $?
  systemctl is-active "$unit.service"

Before, the command timed out with exit status 124 while the service
stayed active. --wait waits for deactivation, but RemainAfterExit=yes
keeps the service active after the command exits.

Follow-up for 2a453c2ee3090e1942bfd962262f3eff0adbfa97