Finish removal of APM mode selection

Just cleaning up the rest of this. The UI was a little weird because the button was gone, but the text was still there.

@LorenzMeier I spoke with the APM Planner folks. They are not doing any sort of big merges across any more. They only cherry pick once and a while. Also the APM Planner branch has diverged quite a bit so trying to keep the two in sync is very difficult and not going to happen. Given the discussion I think we're ok to start disconnecting the APM specific UI and leave APM to be a mavlink only case. Although I still think there are some problems there as well. But those might be fixable.