Cleanup: Remove include/crypto/bn_conf.h.in

Cleanup: Remove include/crypto/bn_conf.h.in

It turns out that this reproduces what's already done conditionally in
include/openssl/configuration.h.in.  Furthermore, include/crypto/bn_conf.h
was only included in crypto/bn/bn_local.h using the same guard as the
definition in include/openssl/configuration.h, which is already included
by crypto/bn/bn_local.h anyway, via include/openssl/opensslconf.h.

Therefore, include/crypto/bn_conf.h.in can be removed, alongside the
construction and inclusion of include/crypto/bn_conf.h.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/28794)