There are two ways to delete a layer. The easiest one is to hit the minus button on the top of the window. That will remove the current selected layer from all frames in the project. That is if you delete the 5th layer while on the third frame, the 5th layer will also be deleted from all other frames.
One way to delete the content of a layer without removing it from other frames, is by just doing a Clear operation on it.
If you want delete any layer that is not the current selected one, you can just swipe that layer to the left, and that layer (in the current frame and all others) will be removed from the project.