small fix for keypress#remove

Hi all, this is just a little 1 line fix. While working on a little app, I tried using keypress#remove and it threw an error. The issue is `remove_key_listener` needs to be handed a `key_listener` (`self`).

I haven't figured out how to spec this since we use doubles for app and the only spec that's needed is one that makes sure app has something valid to remove.

Any ideas?