oniro-image.bbclass: Drop subid backup post rootfs command

Upstream (oe-core) handles now the cleanup directly in the default
sort_passwd post rootfs command.

Merged in oe-core, kirkstone 84b2e9a6c17278cbcc08020aa78759004a7b60bf.
This revision is included in the last oe-core bump.

Related to #772



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