[NETFILTER]: remove NF_CONNTRACK_ENABLED option

Remove the NF_CONNTRACK_ENABLED option. It was meant for a smoother upgrade
to nf_conntrack, people having reconfigured their kernel at least once since
ip_conntrack was removed will have the NF_CONNTRACK option already set.
People upgrading from older kernels have to reconfigure a lot anyway.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>