rootfs.py: uninstall the run-postinsts package if not needed

The run-postinsts package runs post installation scripts
on target if packages request delayed post installations. When
no delayed post installations are found the sysV style scripts
are disabled for the package and hence it did not run on sysV
based systems. However, the package provides systemd service
as well which still ran on systems based on systemd even when
no post installations were found.
Rather than disabling/masking scripts for different initialization
managers we now simply remove/uninstall the run-postinsts package
when no post installations are found to be delayed till runtime.
This is also more aligned with the function (_uninstall_unneeded)
this functionality is triggered through.

Signed-off-by: Awais Belal <awais_belal@mentor.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>