transaction: remove type field, set Version to 0

Two changes being done here, because they require a lot of updates to
tests. Now we're back into version 0 and we only have one type of
transaction.

It also removes GetType and GetScript interops, both are obsolete in Neo 3.

integration/performance_test.go

@@ -77,7 +77,7 @@ func getTX(t *testing.B, wif *keys.WIF) *transaction.Transaction {
 	fromAddressHash, err := address.StringToUint160(fromAddress)
 	require.NoError(t, err)
 
-	tx := transaction.NewInvocationTX([]byte{0x51}, 1)
+	tx := transaction.New([]byte{0x51}, 1)
 	tx.Version = 0
 	tx.Sender = fromAddressHash
 	tx.Attributes = append(tx.Attributes,