[NETFILTER]: nf_conntrack_ipv4: fix incorrect #ifdef config name

The option is named CONFIG_NF_NAT not CONFIG_IP_NF_NAT. Remove the ifdef
completely since helpers also expect defragmented packet even without
NAT.

Noticed by Robert P. J. Day <rpjday@mindspring.com>

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