run-postinsts: remove the init script after a clean start-up
If we enable ipk/deb package back-end, and we have 'package-management'
in our IMAGE_FEATURES, then the /etc/rcS.d/S99run-postinsts would
still exist in our system after a clean start-up.
The initial design for run-postinsts requires the related init script
to be removed if there's no more post-install script left in the system.
