Remove custom EncodeScale/DecodeScale for weakcoin message once go-scale generator issue fixed

[evt]

EncodeScale/DecodeScale manually defined for

// Message defines weak coin message format.
type Message struct {
	Epoch     types.EpochID
	Round     types.RoundID
	Unit      uint64
	MinerPK   []byte
	Signature []byte
}

should be removed and //go:generate scalegen -types Message added once https://github.com/spacemeshos/go-scale/issues/17 has been resolved.