netfilter: nf_conntrack: remove unnecessary function declaration

This patch removes nf_ct_ipv4_ct_gather_frags() method declaration from
include/net/netfilter/ipv4/nf_conntrack_ipv4.h, since it is unused in
the Linux kernel.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>