[NETFILTER]: nf_conntrack: Removes duplicated declarations

These are also in include/net/netfilter/nf_conntrack_helper.h

Signed-off-by: Yasuyuki Kozakai <yasuyuki.kozakai@toshiba.co.jp>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>