netfilter: nft_socket: remove WARN_ON_ONCE on maximum cgroup level

cgroup maximum depth is INT_MAX by default, there is a cgroup toggle to
restrict this maximum depth to a more reasonable value not to harm
performance. Remove unnecessary WARN_ON_ONCE which is reachable from
userspace.

Fixes: 7f3287db ("netfilter: nft_socket: make cgroupsv2 matching work with namespaces")
Reported-by:  <syzbot+57bac0866ddd99fe47c0@syzkaller.appspotmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>