KEYS: x509: remove dead code that set ->unsupported_sig

The X.509 parser always sets cert->sig->pkey_algo and
cert->sig->hash_algo on success, since x509_note_sig_algo() is a
mandatory action in the X.509 ASN.1 grammar, and it returns an error if
the signature's algorithm is unknown.  Thus, remove the dead code which
handled these fields being NULL.

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