Certificate: Remove subject name uniqueness validation

Certificate: Remove subject name uniqueness validation

RFC 6487:

> An issuer SHOULD use a different subject name if the subject's key
> pair has changed (i.e., when the CA issues a certificate as part of
> re-keying the subject.)

Fort's implementation was problematic. The code was comparing the
certificate's subject name and public key to siblings that had
potentially not been validated yet. It seems to me this would make it
possible for attackers to crash FORT (by posting invalid objects) or to
invalidate legitimate objects (by publishing siblings that contained
conflicting subject names and public keys, without having to worry about
the rest of the fields).

This would be somewhat difficult o fix. I asked on the mailing list and
Discord ("RPKI Community"), and it seems the concensus is "don't
validate it." Subject Names don't really matter that much, because
RPKI's primary concern is resource ownership, not identity. Furthermore,
I'm not convinced that chopping off branches off the tree because of a
clumsy key rollover is a good idea.

https://mailarchive.ietf.org/arch/msg/sidrops/mXWbCwh6RO8pAtt7N30Q9m6jUws/

Manually cherry-picked from 0a6a80b558e12304ba0e68c021848e292bfe3ce6.

Hopefully f1xes #86.