rpc: drop support for getassetstate

It's for UTXO assets and it's absent in Neo 3.

pkg/rpc/response/result/asset_state.go

@@ -1,41 +0,0 @@
-package result
-
-import (
-	"github.com/nspcc-dev/neo-go/pkg/core/state"
-	"github.com/nspcc-dev/neo-go/pkg/core/transaction"
-	"github.com/nspcc-dev/neo-go/pkg/encoding/address"
-	"github.com/nspcc-dev/neo-go/pkg/util"
-)
-
-// AssetState wrapper used for the representation of
-// state.Asset on the RPC Server.
-type AssetState struct {
-	ID         util.Uint256          `json:"id"`
-	AssetType  transaction.AssetType `json:"type"`
-	Name       string                `json:"name"`
-	Amount     util.Fixed8           `json:"amount"`
-	Available  util.Fixed8           `json:"available"`
-	Precision  uint8                 `json:"precision"`
-	Owner      string                `json:"owner"`
-	Admin      string                `json:"admin"`
-	Issuer     string                `json:"issuer"`
-	Expiration uint32                `json:"expiration"`
-	IsFrozen   bool                  `json:"is_frozen"`
-}
-
-// NewAssetState creates a new Asset wrapper.
-func NewAssetState(a *state.Asset) AssetState {
-	return AssetState{
-		ID:         a.ID,
-		AssetType:  a.AssetType,
-		Name:       a.GetName(),
-		Amount:     a.Amount,
-		Available:  a.Available,
-		Precision:  a.Precision,
-		Owner:      a.Owner.String(),
-		Admin:      address.Uint160ToString(a.Admin),
-		Issuer:     address.Uint160ToString(a.Issuer),
-		Expiration: a.Expiration,
-		IsFrozen:   a.IsFrozen,
-	}
-}