Remove mode_req_local_alt from Descend mode and handle it in FW position controller

[sfuhrer]

Solved Problem

I recently crashed a VTOL vehicle in FW after disabling both GPS and baro fusion in air, which made it switch from AUTO to TERMINATE mode.

Solution

• remove mode requirement for a valid altitude estimate from DESCEND
• similar to what is done for MC handle invalid v/vz in the FW position controller. For FW we in case of invalid v/vz measurements would likely just want the thrust to be 0, such that the vehicle glides down to the ground while still tracking the loiter (assuming it doesn't stall).

Changelog Entry

For release notes:

Improvement: Remove mode_req_local_alt from Descend mode and handle it in FW position controller

Test coverage

Can be easily tested in SITL: Disable Baro fusion (EKF2_BARO_CTRL), then takeoff in auto mode, then disable GPS fusion --> with current main it goes to Termination mode, with this PR to Descend.