netfilter: conntrack: remove unused code in nf_conntrack_proto_generic.c

L4 protocol helpers for DCCP, SCTP and UDPlite can't be built as kernel
modules anymore, so we can remove code enclosed in
 #ifdef CONFIG_NF_CT_PROTO_{DCCP,SCTP,UDPLITE}_MODULE

Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>