KEYS: x509: remove never-set ->unsupported_key flag

The X.509 parser always sets cert->pub->pkey_algo on success, since
x509_extract_key_data() is a mandatory action in the X.509 ASN.1
grammar, and it returns an error if the algorithm is unknown.  Thus,
remove the dead code which handled this field being NULL.  This results
in the ->unsupported_key flag never being set, so remove that too.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>