Fix no-ui

In a recent commit, a OPENSSL_NO_UI guard was dropped by mistake in
PEM_def_callback(), so this puts it back.  At the same time, that
previous commit got rid of the necessity to have a OPENSSL_NO_STDIO
guard, so we drop that instead.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6096)