For some unknown reason, it does not acquire prefix
via DHCPv6-PD and/or renew the prefix.

Let's disable checksum for now.

Bug report:
https://forum.turris.cz/t/dhcpv6-on-wan-not-working-tos-5-pppoe/12853?u=pepe

Netfilter sysctl defaults were moved into specific kmod-packages from
base-files. When we wanted it to configure in base-files, those values
are overwritten by file sysctl-nf-conntrack.conf provided in the package
kmod-nf-conntrack. We don't need to set it twice and remove it from
base-files and change it only in kmod-nf-conntrack.