Deprecate MozRevocationReasons flag and remove Code from RevokeCertByKeyRequest

[aarongable]

We don't want to have two different revocation codepaths sitting around longer than we have to. Once we've made the policy decision to turn on the MozRevocationReasons flag everywhere, we should remove the old code path asap.

One piece of the old codepath that is not explicitly locked behind the feature flag is the Code field of the RevokeCertByKeyRequest message. Remove that field (and reserve its number) when this deprecation is complete.