rpc: use default Uint160 marshaller for result.NEP5Balance.Asset

To match C# behaviour we should marshal asset hash with `0x` prefix.

pkg/rpc/response/result/nep5.go

@@ -1,8 +1,6 @@
 package result
 
 import (
-	"encoding/json"
-
 	"github.com/nspcc-dev/neo-go/pkg/util"
 )
 
@@ -19,13 +17,6 @@ type NEP5Balance struct {
 	LastUpdated uint32       `json:"lastupdatedblock"`
 }
 
-// nep5Balance is an auxiliary struct for proper Asset marshaling.
-type nep5Balance struct {
-	Asset       string `json:"assethash"`
-	Amount      string `json:"amount"`
-	LastUpdated uint32 `json:"lastupdatedblock"`
-}
-
 // NEP5Transfers is a result for the getnep5transfers RPC.
 type NEP5Transfers struct {
 	Sent     []NEP5Transfer `json:"sent"`
@@ -43,29 +34,3 @@ type NEP5Transfer struct {
 	NotifyIndex uint32       `json:"transfernotifyindex"`
 	TxHash      util.Uint256 `json:"txhash"`
 }
-
-// MarshalJSON implements json.Marshaler interface.
-func (b *NEP5Balance) MarshalJSON() ([]byte, error) {
-	s := &nep5Balance{
-		Asset:       b.Asset.StringLE(),
-		Amount:      b.Amount,
-		LastUpdated: b.LastUpdated,
-	}
-	return json.Marshal(s)
-}
-
-// UnmarshalJSON implements json.Unmarshaler interface.
-func (b *NEP5Balance) UnmarshalJSON(data []byte) error {
-	s := new(nep5Balance)
-	if err := json.Unmarshal(data, s); err != nil {
-		return err
-	}
-	asset, err := util.Uint160DecodeStringLE(s.Asset)
-	if err != nil {
-		return err
-	}
-	b.Amount = s.Amount
-	b.Asset = asset
-	b.LastUpdated = s.LastUpdated
-	return nil
-}