Remove Alpha Channel
This command removes the Alpha channel of the active layer, keeping the
Apha channels of the other layers.
If the active layer is the background layer and if you have not added an
Alpha channel before (then the layer name is in bold letters in the
Layer Dialog), the command is grayed out, inactive.
If the active layer is not the background layer, transparency is replaced
with the background color of the Toolbox.
Activate the Command