sub_jwk when sub is DID

[Sakurann]

Have we agreed to remove sub_jwk? It is not necessary when sub is the DID - would also significantly reduce the complexity of producing the token in that case. But it is needed when sub is jwk thumbprint. Can sub_jwk be must for jwk thumbprint and optional for dids some might want to compare whether verification method from the DID Document matches the kid of sub_jwk, but it's not a must I believe.