core-image-minimal: do not remove packaging files in the rootfs postprocess
Since the removal of PM's meta-data was moved in image.bbclass, after
rootfs_*_do_rootfs() runs, there's no need to remove the files in rootfs
postprocess stage. Also, if the meta-data is removed then there's no way
we can save the postinstalls to /etc/(deb|ipk)-postinsts later.
For RPM, it doesn't really matter since the postinstalls are saved to
/etc/rpm-postinsts as soon as the postinstall fails.
