x86: drop MEMORY_GUARD

x86: drop MEMORY_GUARD

The functions it guards are dead code. Worse, while intended to exist in
debug builds only, as of commit bacbf0cb7349 ("build: convert debug to
Kconfig") they also get compiled in release builds.

The remaining uses in show_stack_overflow() aren't really related to any
memory guarding anymore - with CET-SS support the stacks now get set up
the same in debug and release builds. Drop them as well; there's no harm
providing the information there in all cases.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>