Remove Elevon Config from Plane's Radio Calibration screen

We've recently reworked the ArduPilot wiki page re RC calibration and as part of that @tridge requested that we remove the "Elevon Config" section of MP's radio calibration screen for plane because it hasn't been usable for "half a dozen releases".


Thanks!

Locate the Plane radio calibration screen and the Elevon Config section shown in the issue image. Remove that section while preserving the rest of the calibration screen, then run the relevant Mission Planner UI or Plane calibration flow to confirm it still works.

Written by the indexing model from the issue text.