AP_TECS.cpp: remove line copy error

This was a copy paste error that is duplicated in the correct spot (in line 530)

libraries/AP_TECS/AP_TECS.cpp

@@ -486,7 +486,6 @@ void AP_TECS::_update_height_demand(void)
     {
         _hgt_dem = _hgt_dem_prev - max_sink_rate * 0.1f;
     }
-    _hgt_dem_prev = _hgt_dem;
 
     // Apply first order lag to height demand
     _hgt_dem_adj = 0.05f * _hgt_dem + 0.95f * _hgt_dem_adj_last;