Remove OPENSSL_atexit();

Remove OPENSSL_atexit();

OSSLINAPPL (OpenSSL Is Not A Public Portability Layer)

I blame @nhorman and @sashan, who made me update the adjacent
documentation, and it was sitting there like an unloved and
ignored chronically ill pet with no hope for any future
quality of life.

Since this really does not need an alternative API or a
deprecation stragegy, we can simply remove it. Anyone
who was using this can use libc atexit() if they must.

Reviewed-by: Saša Nedvědický <sashan@openssl.org>
Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
MergeDate: Wed Feb 11 17:37:33 2026
(Merged from https://github.com/openssl/openssl/pull/29874)