r/certificate: subject_alternative_names doesn't need a Set func

[vancluever]

The Set func on this attribute was set to schema.HashString, which is
actually what helper/schema defaults a TypeSet with an element of
this type to anyway.

Removed it and tested on a config with a few SANs, no change.

[vancluever]

Hmm, I think I'm just going to leave this one. It looks like SerializeValueForHash behaves slightly differently than a simple HashString, as it writes a semicolon at the end of the pre-hash buffer, whereas HashString just converts the value as-is without any more manipulation.

I don't know why my manual test didn't give a diff, but the honestly is not worth it enough to mess with people's states.