patches/openwrt: nf-conntrack: improve defaults

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.