Some platforms don't have pthread_atfork

Some platforms don't have pthread_atfork

We've had a report of a linker failure on some platforms (this one was
linux ARM) that apparently did not have pthread_atfork. It's strange that
this has not been reported before but the simplest solution is just to
remove this from the library since it isn't really used anyway.

Currently it is called to set up the fork handlers OPENSSL_fork_prepare,
OPENSSL_fork_parent and OPENSSL_fork_child. However all of those functions
are no-ops. This is a remnant from earlier code that got removed. We can
safely remove it now.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Tim Hudson <tjh@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17842)