build: remove unused code spotted by the deadcode linter

backend/cache/storage_persistent.go

@@ -1070,10 +1070,3 @@ func itob(v int64) []byte {
 func btoi(d []byte) int64 {
 	return int64(binary.BigEndian.Uint64(d))
 }
-
-// cloneBytes returns a copy of a given slice.
-func cloneBytes(v []byte) []byte {
-	var clone = make([]byte, len(v))
-	copy(clone, v)
-	return clone
-}