Remove timestamp from tx

[thanhnguyennguyen]

In the past, we tried to add timestamp to tx to make sure that all matching transactions are sorted by time.
Since we force push matching transaction to block (if it exists and DOES NOT push to tx pool), every block has at most 1 matching transaction.
Timestamp is not necessary now, we should remove it

[thanhnguyennguyen]

We are going to use this timestamp to update createdAt updatedAt to mongodb #778
So keep it