Deleting a cell is very simple. To delete a cell, drag the cell off of the designer and wait until a trash car appears on the left side of the screen.
Once the trash can appears, keep dragging the cell over to the black area until it turns red. Once the trash can area turns red, you are safe to let go of the cell. The cell will then be removed from the designer.
Be sure to save your changes, or the cell will re-appear.
If you accidentally delete a cell, you can get it back simply by either refreshing the page, or clicking the discard changes button directly above the themes icon on the lower right of the screen.
See figure 1A for further reference.