To delete default price, you need to go to Inventory > My Inventory > click on the item > select Listing descriptions tab. Select the default price and click the Delete button.        

Please Note! There is no option to delete Default Price by Channel in bulk with the import. Instead, you can adjust it to match the current retail price of the product. To do so you need to get a CSV file with the SKU's and current prices.

Easy option to get this file is to use  Query Data report >  Stock Item (no levels) report type.

Download the report and re-import this file with the Inventory Import. Map the Retail Price column to the Price by Channel (Default) column in the inventory tool's mapping screen.