cgroup: Drop sock_cgroup_classid() dummy implementation

cgroup: Drop sock_cgroup_classid() dummy implementation

The semantic of returning 0 is unclear when !CONFIG_CGROUP_NET_CLASSID.
Since there are no callers of sock_cgroup_classid() with that config
anymore we can undefine the helper at all and enforce all (future)
callers to handle cases when !CONFIG_CGROUP_NET_CLASSID.

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Link: https://lore.kernel.org/r/Z_52r_v9-3JUzDT7@calendula/
Acked-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>