windowManager: Remove four finger app switch gesture

This gesture is not implemented very well, it's basically a hack. It also doesn't work all the time and using it sometimes leaves the shell in a broken state.

This could be implemented properly with the new gesture actions (mutter!424 (closed)), I think the gesture doesn't make much sense though and should just be removed. Feel free to add the "Needs Design" label in case we want to keep it or discuss an alternative.