compiler|transaction: remove transaction.GetUnspentCoins support

It's useless. Even though there is Neo.Transaction.GetUnspentCoins syscall
that can be used, its return type is an interop structure that's not accepted
by any other syscall, so you can't really do anything with it. And there is no
such interface for the .net Framework.

docs/runtime.md

@@ -504,12 +504,6 @@ GetReferences(t Transacfion) interface{}
 ```
 Returns the references of the given transaction.
 
-#### GetUnspentCoins
-```
-GetUnspentCoins(t Transacfion) interface{} 
-```
-Returns the unspent coins of the given transaction.
-
 #### GetOutputs
 ```
 GetOutputs(t Transacfion) []output.Output 

pkg/compiler/syscall.go

@@ -65,14 +65,13 @@ var syscalls = map[string]map[string]string{
 		"GetTransaction":      "Neo.Block.GetTransaction",
 	},
 	"transaction": {
-		"GetHash":         "Neo.Transaction.GetHash",
-		"GetType":         "Neo.Transaction.GetType",
-		"GetAttributes":   "Neo.Transaction.GetAttributes",
-		"GetInputs":       "Neo.Transaction.GetInputs",
-		"GetOutputs":      "Neo.Transaction.GetOutputs",
-		"GetReferences":   "Neo.Transaction.GetReferences",
-		"GetUnspentCoins": "Neo.Transaction.GetUnspentCoins",
-		"GetScript":       "Neo.InvocationTransaction.GetScript",
+		"GetAttributes": "Neo.Transaction.GetAttributes",
+		"GetHash":       "Neo.Transaction.GetHash",
+		"GetInputs":     "Neo.Transaction.GetInputs",
+		"GetOutputs":    "Neo.Transaction.GetOutputs",
+		"GetReferences": "Neo.Transaction.GetReferences",
+		"GetScript":     "Neo.InvocationTransaction.GetScript",
+		"GetType":       "Neo.Transaction.GetType",
 	},
 	"asset": {
 		"Create":       "Neo.Asset.Create",

pkg/interop/transaction/transaction.go

@@ -33,12 +33,6 @@ func GetReferences(t Transaction) []interface{} {
 	return []interface{}{}
 }
 
-// GetUnspentCoins returns the unspent coins for the given transaction.
-// FIXME: What is the correct return type for this?
-func GetUnspentCoins(t Transaction) interface{} {
-	return 0
-}
-
 // GetInputs returns the inputs of the given transaction.
 func GetInputs(t Transaction) []input.Input {
 	return []input.Input{}