Complete SignedEpoch JSON migration in v0.19.0

[xenowits]

🎯 Problem to be solved

Complete signed epoch JSON migration in v0.19.0. See the migrations doc.

🛠️ Proposed solution

Refactor SignedEpoch.UnmarshalJSON() to the following:

// UnmarshalJSON unmarshalls SignedEpoch.
func (s *SignedEpoch) UnmarshalJSON(b []byte) error {
	var resp signedEpochJSON
	if err := json.Unmarshal(b, &resp); err != nil {
		return errors.Wrap(err, "unmarshal signed epoch")
	}

	s.Epoch = resp.Epoch
	s.Signature = resp.Signature

	return nil
}

Also, remove legacySignedEpochJSON type in the same file.