Unfortunately, there is no way to delete downloaded banking transactions in new QuickBooks Online.
Select the transactions (you like to delete) and then use the “Exclude Selected” option under Batch Actions. It will move from “New Transactions” column to “Excluded” column.
QBO developers claim, it will sit there forever harmlessly, but I do not buy it.
If you feel this “delete” feature need to be added in future revision, please add your voice here. I’ve already done it in this past.