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>