sysctl: Remove the now unused ctl_table parent field.

While useful at one time for selinux and the sysctl sanity
checks those users no longer use the parent field and we can
safely remove it.

Inspired-by: Lucian Adrian Grijincu <lucian.grijincu@gmil.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/linux/sysctl.h

@@ -1016,7 +1016,6 @@ struct ctl_table
 	int maxlen;
 	umode_t mode;
 	struct ctl_table *child;	/* Deprecated */
-	struct ctl_table *parent;	/* Automatically set */
 	proc_handler *proc_handler;	/* Callback for text formatting */
 	struct ctl_table_poll *poll;
 	void *extra1;