netfilter: nftables: remove unnecessary spin_lock_init()

The spinlock nf_tables_destroy_list_lock is initialized statically.
It is unnecessary to initialize by spin_lock_init().

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>