Add `deletelayer!` api?

[findmyway]

Now we support push(first)layer! and pop(first)layer!. Sometimes, we added a layer with push(first)layer! and then we want to remove this layer later. Blindly calling pop(first)layer! may remove some other layer that was added later.

[quinnj]

Closing as stale; as of the 2.0 release, the layer apis have been removed all together