inputpanel: ignore requested output

While there's specific hardware where the IM could benefit from requesting to be shown on a specific output, it effectively never has enough information to choose a useful output - and the protocol doesn't allow setting a null output to indicate that the compositor should do the choice either. To avoid showing the OSK on the wrong output, always put it on the active output and ignore what the IM client requests.

Without this, Maliit sometimes ends up on the external output with my laptop, and stays there... which is not ideal