Native: do not remove malicious conflict records during OnPersist

It's OK to keep them and save O(n) operations during OnPersist. The
storage taken by these malicious conflict records is properly paid
anyway. See the discussion under #2913 (comment).

Signed-off-by: Anna Shaleva <shaleva.ann@nspcc.ru>

src/Neo/SmartContract/Native/LedgerContract.cs

@@ -47,9 +47,9 @@ internal override ContractTask OnPersist(ApplicationEngine engine)
             engine.Snapshot.Add(CreateStorageKey(Prefix_Block).Add(engine.PersistingBlock.Hash), new StorageItem(Trim(engine.PersistingBlock).ToArray()));
             foreach (TransactionState tx in transactions)
             {
-                // Remove possible previously saved malicious conflict records for the transaction (if any).
-                foreach (var (key, _) in engine.Snapshot.Find(CreateStorageKey(Prefix_Transaction).Add(tx.Transaction.Hash).ToArray()))
-                    engine.Snapshot.Delete(key);
+                // It's possible that there are previously saved malicious conflict records for this transaction.
+                // These records are garbage in fact and can be removed in a separate flow, but at the same time
+                // it won't hurt to keep them.
 
                 engine.Snapshot.Add(CreateStorageKey(Prefix_Transaction).Add(tx.Transaction.Hash), new StorageItem(tx));