sctp: remove unnecessary NULL checks in sctp_enqueue_event()

After commit 178ca044

 ("sctp: Make sctp_enqueue_event tak an
skb list."), skb_list cannot be NULL.

Detected using the static analysis tool - Svace.
Signed-off-by: Alexey Kodanev <aleksei.kodanev@bell-sw.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20221019180735.161388-3-aleksei.kodanev@bell-sw.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>