rootfspostcommands.py: Cleanup subid backup files generated by shadow-utils

rootfspostcommands.py: Cleanup subid backup files generated by shadow-utils

When creating users, shadow-utils might create backup files for
subordinate ID files (subid, subgid). Make sure we clean them up
similarly to the other backup files shadow-utils creates.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>