Remove os-release diversions once rootfs creation is finished

This is necessary to avoid overwrites while packages are being installed when the chroot is being built up. Once it's done, it's no longer necessary.

This fixes a problem where Calamares rsyncs the whole read-only live rootfs into an installed system, which then is supposed to get updated, and the divert stops the file from being updated as expected, which was never intended.

Follow-up for 24bfe093