Remove HEADER_X509_H include detector from apps

The HEADER_X509_H check is redundant, because <openssl/x509.h>
is already included.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
(Merged from https://github.com/openssl/openssl/pull/9365)