To delete a stock from your Watchlist, simply click on the 'X' button associated with the stock you would like to delete. You will find the 'X' button on the right-hand side of the row.
After clicking the 'X', the stock will be removed from your Watchlist. You will need to refresh your screen to see this change reflected on your Watchlist.
If you did not mean to click the 'X,' you will have the opportunity to add that stock back by hitting the red plus sign.