The Delete Layer Mask command deletes the
active layer's layer mask, without modifying the active layer itself. If
the active layer does not have a layer mask, the menu entry is
insensitive and grayed out.
7.24.1. Activating the Command
You can access this command from the image menubar through
Layer → Mask → Delete Layer Mask,
or from the pop-up menu you get by right-clicking on the active
layer in the Layers Dialog.