sctp: remove unnecessary NULL check in sctp_ulpq_tail_event()

After commit 013b96ec

 ("sctp: Pass sk_buff_head explicitly to
sctp_ulpq_tail_event().") there is one more unneeded check of
skb_list for 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/20221019180735.161388-2-aleksei.kodanev@bell-sw.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>