rootfs: remove unnecessary and potentially harmful force flags from opkg call

Especially --force-overwrite and --force-depends will often lead to broken
images; it's better to fail the build in such cases than to silently ignore
the errors.

Instead, ignore errors in the per-device rootfs opkg remove command, so
the build doesn't break when packages can't be removed because of
dependencies.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>