Update blockage migration to remove hack for zkp bug

The blockage migration protocol included a hack for one of the credential attributes (Invitations remaining) that differed from the spec outlined in the paper. The hack involved blinding the number of invitations a user that is migrating to new bridges after a blockage would receive from the server. This does not need to be blinded because the server determines the number of invitations based on the level that a user will be reduced to and the level is revealed in both presentation and issuing of the credential. This hack was necessary in the case that the number of invitations was reduced to 0 and this value (non-blinded) was rejected as the identity, which is a bug. Upon fixing the bug in zkp!1 we can remove the hack.