Remove stack guard option from hardening menu.

Since kib's change the stack guard is now ON by default,
this option in hardening menu of bsdinstall is no longer needed.

Submitted by: Bartlomiej Rutkowski <robak@FreeBSD.org>
Reviewed by: bapt
Approved by: bapt
MFC after: 1 day
Sponsored by: Pixeware LTD
Differential Revision: https://reviews.freebsd.org/D11686