Magnetic Compass: Simplify

[hbeni]

Simplify magnetic compass code to use c++ based pitch/roll values for the disc

See:

• https://sourceforge.net/p/flightgear/codetickets/2904/
• https://github.com/HHS81/c182s/pull/601

(:warning: this is exclusive to 2020.4 only, not to be backported)

[wlbragg]

@hbeni Same question here, did something chang c++ side that indicated we need to get rid of the filter for this. I don't know who or why these filters are being used?

[hbeni]

The same comment aplies; in short:

did something chang c++ side that indicated we need to get rid of the filter for this

=> yes