netfilter: nf_ct_expect: nf_ct_expect_insert() returns void

Because nf_ct_expect_insert() always succeeds now, its return value can
be just void instead of int. And remove code that checks for its return
value.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>