oniro-image.bbclass: Add rootfs function for cleaning up subid backup files

Podman requires subids configuration[1] but when creating them, shadow
does it using backup files. Clean them up too if available.

[1] https://github.com/containers/podman/blob/main/docs/tutorials/rootless_tutorial.md#etcsubuid-and-etcsubgid-configuration



Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>