cgroup: remove cftype->release()

Now that pidlist files don't use cftype->release(), it doesn't have
any user left.  Remove it.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -499,8 +499,6 @@ struct cftype {
 	 * kick type for multiplexing.
 	 */
 	int (*trigger)(struct cgroup_subsys_state *css, unsigned int event);
-
-	int (*release)(struct inode *inode, struct file *file);
 };
 
 /*

kernel/cgroup.c

@@ -2448,12 +2448,9 @@ static int cgroup_file_open(struct inode *inode, struct file *file)
 static int cgroup_file_release(struct inode *inode, struct file *file)
 {
 	struct cfent *cfe = __d_cfe(file->f_dentry);
-	struct cftype *cft = __d_cft(file->f_dentry);
 	struct cgroup_subsys_state *css = cfe->css;
 	int ret = 0;
 
-	if (cft->release)
-		ret = cft->release(inode, file);
 	if (css->ss)
 		css_put(css);
 	if (file->f_op == &cgroup_seqfile_operations)