Proper vertical climb/descend speed calculation
[rotorcraft] RC_CLIMB cleanup

- add GUIDANCE_V_CLIMB_RC_DEADBAND (in pprz units, default MAX_PPRZ/10)
  - GUIDANCE_V_RC_CLIMB_DEAD_BAND is obsolete
- GUIDANCE_V_MAX_RC_CLIMB_SPEED defaults to GUIDANCE_V_REF_MIN_ZD
- GUIDANCE_V_MAX_RC_DESCENT_SPEED defaults to GUIDANCE_V_REF_MAX_ZD
[conf][airframes] remove obsolete RC_CLIMB_COEF and RC_CLIMB_DEAD_BAND