Remove revocation_time/revocation_reason on unrevoke

[reaperhulk]

Got this from a user:

"Not sure if R509::Validity::Redis::Writer should remove "revocation_time" and "revocation_reason" key on unrevoke. At least it should remove the values, what do you think?"

I can't remember if we had a good reason for leaving that data present when unrevoking a certificate? Was it to keep a record that the certificate had been revoked in the past? If you revoke the certificate again these fields are overwritten with new values, so they're really just dangling in the data store.