Remove driver and "if statements" checking if driver==nuopc in build-namelist

There's some checking on what driver is being used in build-namelist, but right now the only valid option is nuopc. So this just adds needless complexity. If a different driver were to be added as an option it would likely need to be handled in completely different ways, so there is no reason to keep these current checks.

So as we see see "if statements" for the driver, we might as well remove them. A small PR to b4b-dev could also be done to remove it entirely.