net: ieee802154: remove an unnecessary null pointer check

llsec_parse_seclevel has the null pointer check at its begining. Compared
with nl802154_add_llsec_seclevel, nl802154_del_llsec_seclevel has a
redundant null pointer check of info->attrs[NL802154_ATTR_SEC_LEVEL]
before llsec_parse_seclevel.

Fix this issue by removing the null pointer check in
nl802154_del_llsec_seclevel.

Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Link: https://lore.kernel.org/r/20230308083231.460015-1-dzm91@hust.edu.cn
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>