cgroup.h: remove some functions that are now gone

cgroup_lock() and cgroup_unlock() are now no longer exported, so fix
cgroup.h to not declare them if CONFIG_CGROUPS is not enabled.
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup.h

@@ -840,8 +840,6 @@ static inline void cgroup_fork(struct task_struct *p) {}
 static inline void cgroup_post_fork(struct task_struct *p) {}
 static inline void cgroup_exit(struct task_struct *p, int callbacks) {}
 
-static inline void cgroup_lock(void) {}
-static inline void cgroup_unlock(void) {}
 static inline int cgroupstats_build(struct cgroupstats *stats,
 					struct dentry *dentry)
 {