Quick way to delete unwanted areas for all layers

Hi,

I’m trying to remove unwanted areas from a sprite with multiple layers so that I’m left with only the sprite areas that I want. Unfortunately, resizing the canvas does not remove the unwanted areas, nor does cropping the sprite (if I expand the canvas again, the unwanted areas do not appear to be deleted). How do I delete all unwanted areas from multiple layers in one go?

Thanks.