Fix bogus check for EVP_PKEY_supports_digest_nid() in check_cert_usable()

Fix bogus check for EVP_PKEY_supports_digest_nid() in check_cert_usable()

In commit 2d263a4a73 ("Honour mandatory digest on private key in
has_usable_cert()" I added two checks for the capabilities of the
EVP_PKEY being used. One of them was wrong, as it should only be
checking the signature of the X.509 cert (by its issuer) against the
sigalgs given in a TLS v1.3 signature_algorithms_cert extension.

Remove it and provide the code comments which, if they'd been present
in the first place, would hopefully have prevented the mistake.

Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9672)