Revert "base-files: sysupgrade: always setup overlay when creating backup"

Revert "base-files: sysupgrade: always setup overlay when creating backup"

This reverts commit 4fa9aaf0bed984d200b3c48d1cc81fca7847c394.

That seemed like a good idea allowing us to include any runtime
generated file in archive. Unfortuantely it broke backups with files
from mounted directories.

When mounting overlay with / as lowerdir its mounts don't propagete in
the mountpoint. That resulted in empty directories:
/tmp/overlay.XXXXXX/backup/tmp/
/tmp/overlay.XXXXXX/backup/var/
/tmp/overlay.XXXXXX/backup/dev/
/tmp/overlay.XXXXXX/backup/proc/
etc.

As some platforms / users try to backup files like /var/dhcp.leases or
/boot/cmdline.txt it means we can't use that solution.

Link: http://lists.openwrt.org/pipermail/openwrt-devel/2024-February/042320.html
Link: https://lore.kernel.org/linux-fsdevel/67bb0571-a6e0-44ea-9ab6-91c267d0642f@gmail.com/T/#u
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>