Radio Calibration page needs an update

The radio calibration page in MissionPlanner needs an update. There are a few key problems:
- we should remove the ancient "Elevon Config" section at the bottom, that has not been relevant for a very very long time.
- the method of getting correct reversals for roll, pitch and yaw are very bad. It needs redoing in a way that is much clearer

The reversal problem is particularly bad. For an inexperienced user they really have no idea if their pitch is correctly reversed or not. This leads to users flying with inverted pitch control. I think we need a re-think of the UI to make it clear that for pitch a non-reversed pitch means that higher input values should be pitch up (which means pulling back on the pitch stick). Maybe a diagram showing the stick positions with reversal taken into account? (would need mode1 and mode2 options?)

Start by locating the Mission Planner radio calibration page and reviewing the existing Elevon Config section and roll, pitch, and yaw reversal controls. Clarify the proposed UI and mode 1/mode 2 behavior before changing anything; done means the obsolete section is removed and reversal guidance makes correct pitch direction unambiguous.

Written by the indexing model from the issue text.