There are two main methods to erase (using any number of tools, though):
1. Overwrite parts you don't want with black or whatever you're using as you're background. Remember that right-clicking acts as a dropper, which makes this pretty easy--just right-click in an empty space and start painting over what you want to erase (or use other tools to erase different shapes, e.g. rect, oval, lines, floodfill, rather than doing it cell-wise).
2. Use the Cut tool (ctrl-x) to remove areas, which leaves transparency when used on upper layers, or turns the base layer black.
There's no need for a dedicated erase tool since it would just do the same thing as the above. I should probably put this in the manual, though, because it's been asked a few times over the years
