systemd: clear telinit path

Since we don't enable sysv any support, it makes no sense to set the
path to telinit either.

The path we were setting was anyway wrong: we set a path into
TARGET_DIR, but this path is only used at runtime, on the target, where
TARGET_DIR doesn't exist (it should have been /usr/sbin/telinit).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>