netfilter: nft_socket: remove WARN_ON_ONCE with huge level value

syzbot managed to reach this WARN_ON_ONCE by passing a huge level
value, remove it.

  WARNING: CPU: 0 PID: 5853 at net/netfilter/nft_socket.c:220 nft_socket_init+0x2f4/0x3d0 net/netfilter/nft_socket.c:220

Reported-by:  <syzbot+a225fea35d7baf8dbdc3@syzkaller.appspotmail.com>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>