Once your create a + Assessment, it is moved to the Assigned Assessment module for the class it was assigned.
In order to delete a + Assessment, select Assessments from the left of the page.
Select "Assigned" from the submenu to the left and then choose the desired class from the center drop down.
Click the ellipsis, by hovering over the + Assessment that you would like to remove and select unassign.
After confirming to unassign the + Assessment, you will no longer see this assessment in your assigned gradebook and it will be deleted.