Deals cannot be deleted in V2. If you want to clean up or remove old deals from the pipeline view, you can change the status to "On Hold".
Select the deal you no longer wish to see in the Deal Pipeline. Next to the lead name click the "Active" drop down. Change this to "On Hold"
A status change box will populate. Select drop down for reason if required and add notes. Click Save.
Once saved, the deal will be placed in "On Hold" status and will no longer show in the pipeline view.
To view deals that are on hold, apply a filter to the deal pipeline that shows deals with the on hold status.