task_stack.h: remove obsolete __HAVE_ARCH_KSTACK_END check

Remove __HAVE_ARCH_KSTACK_END as it has been obsolete since removal of
metag architecture in v4.17.

Link: https://lkml.kernel.org/r/20250403-kstack-end-v1-1-7798e71f34d1@linaro.org


Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Link: https://lore.kernel.org/20240311164638.2015063-2-pasha.tatashin@soleen.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>