pmb.config: do not prompt for nonfree fw/userland (MR 2255)

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.