cgroup: remove unused dummy cgroup_fork_callbacks()

5edee61ede ("cgroup: cgroup_subsys->fork() should be called after the
task is added to css_set") removed cgroup_fork_callbacks() but forgot
to remove its dummy version for !CONFIG_CGROUPS.  Remove it.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Herton Ronaldo Krzesinski <herton.krzesinski@canonical.com>