vm: remove iterator/enumerator Concat API

Follow neo-project/neo#2170.

pkg/interop/iterator/iterator.go

@@ -19,17 +19,6 @@ func Create(items interface{}) Iterator {
 	return Iterator{}
 }
 
-// Concat concatenates two given iterators returning one that will range on
-// a first and then continue with b. Iterator positions are not reset for a
-// and b, so if any of them was already advanced by Next the resulting
-// Iterator will point at this new position and never go back to previous
-// key-value pairs. Concatenated iterators also remain completely independent
-// in results they return, so if both contain the same key you'll receive this
-// key twice when iterating. This function uses `System.Iterator.Concat` syscall.
-func Concat(a, b Iterator) Iterator {
-	return Iterator{}
-}
-
 // Key returns iterator's key at current position. It's only valid to call after
 // successful Next call. This function uses `System.Iterator.Key` syscall.
 func Key(it Iterator) interface{} {