VehicleAirData: remove unused air temperature handling

[sfuhrer]

Solved Problem

_air_temperature_celsius is not used anywhere anymore.

Solution

Remove it along and all related.

Changelog Entry

For release notes:

Feature/Bugfix XYZ
New parameter: XYZ_Z
Documentation: Need to clarify page ... / done, read docs.px4.io/...

Alternatives

Should we re-visit the temperature handling? Does it make sense to use the baro temperature for the rho calculation when we also would have the (usually) more accurate temperature from differential_pressure? How do we handle too high baro temperature readings due to board heat up (for differential pressure we constantly subtract 5 deg, see PCB_TEMP_ESTIMATE_DEG). Is there a smarter way? Should we even leave it up to user to specify source of temperature?