Ardupilot: Plane: Allow Pre-Arm check pass without compasses

[Naterater]

Bug report

Issue details
When using plane without a compass, the INS pre-arm check will not pass... according to https://discuss.ardupilot.org/t/ahrs-not-healthy-on-arm/50239/

I recommend NOT disabling INS pre-arm checks in order to pass; we should allow passing, but maybe write a status text indicating that no compasses are enabled when arming.

Version
Plane 4.0

Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X] Plane
[ ] Rover
[ ] Submarine

Airframe type
All

Hardware type
Presumably Matek

Logs
None given in the discuss topic.

[kd0aij]

I think setting COMPASS_USE to zero is a reasonable way to indicate that no compass is present, and in that case, the INS prearm check should pass.

[peterbarker]

@kd0aij That's current behaviour, isn't it?

So can we close this?

[kd0aij]

If @Naterater agrees...