Remove last references to DEBUG_SAFESTACK

The DEBUG_SAFESTACK preprocessor define is obsolete since 2008
when the non-safestack code was removed by commit 985de863

.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9070)

(cherry picked from commit d4f63f1c)