Plane: Do not apply minimum throttle during SLT VTOL airbrake

[Georacer]

Summary

This PR ensures that the bool SLT_Transition::active_frwd() check does not return true during a back-transition airbrake.

Details

The existing code would cause active_frwd() to return true during a back transition.
In turn, servos.cpp:553 would trigger:

#if HAL_QUADPLANE_ENABLED
    if (quadplane.in_frwd_transition()) {

and would set the minimum throttle to the forwards transition throttle: THR_TRIM or TKOFF_THR_MIN.

This would lead to undesired increase in airspeed and eventually altitude during the back transition.

The TECS minimum throttle can be used as a proxy to see the effect of that setting.
This is a snapshot during the airbrake phase.

Before:

After: