pmb.config: do not prompt for nonfree fw/userland

This drops the prompt for using non-free firmware in images. The logic for searching/installing non-free fw subpackages for devices is kept, and will always be installed. This is to support the many device packages in pmaports that still have nonfree-firmware subpackages. Going forward, device packages can list firmware in depends= (for required fw) or pmb_recommends (for optional fw).

nonfree-userland wasn't used in pmaports as far as I could find.