frr: T6746: handle "system ip" and "system ipv6" with FRRender class

FRR 10.2 will use "[no] ip forwarding" and "[no] ipv6 forwarding" to enable or
disable IP(v6) forwarding. We no longer rely on sysctl as this was overridden
by FRR later on.

Remove code path for sysctl setting and solely rely on FRR.