Remove dependency on `openssl/conf_api.h`

None of the functions defined in this header are actually used in Ruby.
Fixes build against boringssl that does not have this file.
Closes: https://github.com/ruby/ruby/pull/2210

ext/openssl/ossl.h

@@ -27,7 +27,6 @@
 #include <openssl/hmac.h>
 #include <openssl/rand.h>
 #include <openssl/conf.h>
-#include <openssl/conf_api.h>
 #include <openssl/crypto.h>
 #if !defined(OPENSSL_NO_ENGINE)
 #  include <openssl/engine.h>