ieee802154: Remove WARN_ON() in cfg802154_pernet_exit()

ieee802154: Remove WARN_ON() in cfg802154_pernet_exit()

There's no need to call WARN_ON() in cfg802154_pernet_exit(), since
every point of failure in cfg802154_switch_netns() is covered with
WARN_ON(), so remove it.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 66e5c2672cd1 ("ieee802154: add netns support")
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Ivan Abramov <i.abramov@mt-integration.ru>
Link: https://lore.kernel.org/20250403101935.991385-4-i.abramov@mt-integration.ru
Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>