amt: remove unnecessary skb pointer check

The skb pointer will be checked in kfree_skb(), so remove the outside check.

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Taehee Yoo <ap420073@gmail.com>
Link: https://lore.kernel.org/r/20220818093114.2449179-1-yangyingliang@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>