Request compassmot for the plane firmware

First raised in 2015 and closed with reason "compassmot will soon be replaced by an in-flight auto-learning scheme for both platforms."
https://github.com/ArduPilot/ardupilot/issues/2204

### Feature request
I am receiving a lot of compass errors due to the small platform that I'm building on.
I am unable to move hi-current wires further from the compasses nor is shielding proving to be effective.
Auto learns compasses is not resolving the issue.

**Describe the solution you'd like**
Please add compass mot calibration to the plane firmware

**Describe alternatives you've considered**
Relaying out wires, wire shielding, inflight calibration, turning off onboard compasses, and the dreaded flying on 1 compass

**Platform**
[ ] All
[ ] AntennaTracker
[ ] Copter
[x ] Plane
[ ] Rover
[ ] Submarine

Begin by reviewing linked issue #2204 and the Plane firmware's current compass-calibration behavior. Confirm whether compassmot is still the intended solution given the referenced in-flight auto-learning scheme; completion requires an agreed Plane-specific implementation and validation criteria.

Written by the indexing model from the issue text.