Revert "mavlink: GLOBAL_POSITION_INT send without lat/lon availability"

[LorenzMeier]

Reverts #15308

85% of PX4 contributors are working with commercial vendors. So our contributors expect commercial quality out of this open source project, not some hobby entertainment. The fact that APM has been doing this for years doesn't make it a valid approach. Entering magic values into a valid range is against best practices. It is unlikely that a lat / lon of 0/0 ever hits a real system - yes. But if I were to review the PX4 codebase and found stuff like that I would loose trust into the implementers because very apparently they are too lazy to implement things correctly and have all sorts of magic easter eggs hidden.

And magic values are fine until they are not. And it is very simple to objectify this: If a unit test that does an input data sweep would fall over, then it is a bad change. In this case a sweep across all possible coordinates would generate an incorrect result here, so it is a bad change.

[dagar]

I don't disagree, but I view this as a Mavlink issue + workaround and certainly not tolerating magic values in general.

@DonLakeFlyer when you're available let's discuss the original issue you were trying to solve.