netfilter: nf_defrag_ipv4: Drop redundant ip_send_check()

Since commit 0848f642

 ("inet: frags: fix defragmented packet's IP
header for af_packet"), ip_send_check() would be called twice for
defragmentation that occurs from netfilter ipv4 defrag hooks. Remove the
extra call.

Signed-off-by: Joe Stringer <joe@ovn.org>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>