rootfs.py: change the logic in _uninstall_unneeded

rootfs.py: change the logic in _uninstall_unneeded

Previously, if we have 'package-management' and 'read-only-rootfs'
both in IMAGE_FEATRUES, we would meet the following error at system
start-up.

	rm: can't remove '/etc/rcS.d/S99run-postinsts': Read-only file system

However, what's really expected is that when there's no postinstall
script at system start-up, the /etc/rcS.d/S99run-postinsts should not
even be there.

Whether or not to remove the init script symlinks to run-postinsts should
not depend on whether we have 'package-management' in IMAGE_FEATURES; rather,
it should only depend on whether we have any postinstall script left to run
at system start-up.

This patch changes the _uninstall_unneeded function based on the logic
stated above.

[YOCTO #6257]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>