netfilter: nf_nat: remove obsolete check in nf_nat_mangle_udp_packet()

The packet size check originates from a time when UDP helpers could
accidentally mangle incorrect packets (NEWNAT) and is unnecessary
nowadays since the conntrack helpers invoke the NAT helpers for the
proper packet directly.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>