Plane: is_crashed flag gets reset too easily

This behavior is excessively paranoid about clearing the flag so now it's extra sticky. You can only clear the is_crashed flag when:
- changing modes
- starting to execute a takeoff wp (if mission/index gets reset while still in auto)
- while in takeoff and throttle is suppressed it's held false

behavior that was removed:
- clear flag when starting to execute any nav cmd (reached next wp)
- if while crashed, you "start flying again" (non-sticky)

ArduPlane/commands_logic.cpp

@@ -22,9 +22,6 @@ bool Plane::start_command(const AP_Mission::Mission_Command& cmd)
         // except in a takeoff
         auto_state.takeoff_complete = true;
 
-        // if we are still executing mission commands then we must be traveling around still
-        crash_state.is_crashed = false;
-
         // if a go around had been commanded, clear it now.
         auto_state.commanded_go_around = false;
 

ArduPlane/is_flying.cpp

@@ -207,7 +207,6 @@ void Plane::crash_detection_update(void)
     if (!crashed) {
         // reset timer
         crash_state.debounce_timer_ms = 0;
-        crash_state.is_crashed = false;
 
     } else if (crash_state.debounce_timer_ms == 0) {
         // start timer